Support `:json` type and `via` attribute option for Alert Management Payload parser

What does this MR do?

This MR adds support for inline JSON (type: :json) and the ability to lookup up values using a different attribute/method (via: :method) so we can parse "almost" AWS CloudWatch payloads like:

          {
            "Record": {
              "EventSource": "aws:sns",
              "EventVersion": "1.0",
              "EventSubscriptionArn": "arn:aws:sns:eu-west-1:000000000000:cloudwatch-alarms:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
              "Sns": {
                "Type": "Notification",
                "MessageId": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
                "TopicArn": "arn:aws:sns:eu-west-1:000000000000:cloudwatch-alarms",
                "Subject": "ALARM: \"Example alarm name\" in EU - Ireland",
                "Message": "{\"AlarmName\":\"Example alarm name\",\"AlarmDescription\":\"Example alarm description.\",\"AWSAccountId\":\"000000000000\",\"NewStateValue\":\"ALARM\",\"NewStateReason\":\"Threshold Crossed: 1 datapoint (10.0) was greater than or equal to the threshold (1.0).\",\"StateChangeTime\":\"2017-01-12T16:30:42.236+0000\",\"Region\":\"EU - Ireland\",\"OldStateValue\":\"OK\",\"Trigger\":{\"MetricName\":\"DeliveryErrors\",\"Namespace\":\"ExampleNamespace\",\"Statistic\":\"SUM\",\"Unit\":null,\"Dimensions\":[],\"Period\":300,\"EvaluationPeriods\":1,\"ComparisonOperator\":\"GreaterThanOrEqualToThreshold\",\"Threshold\":1.0}}",
                "Timestamp": "2017-01-12T16:30:42.318Z",
                "SignatureVersion": "1",
                "Signature": "Cg==",
                "SigningCertUrl": "https://sns.eu-west-1.amazonaws.com/SimpleNotificationService-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.pem",
                "UnsubscribeUrl": "https://sns.eu-west-1.amazonaws.com/?Action=Unsubscribe&SubscriptionArn=arn:aws:sns:eu-west-1:000000000000:cloudwatch-alarms:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
                "MessageAttributes": {}
              }
            }
          }

⚠ It's almost because the real AWS CloudWatch payloads has a list of Records which isn't supported but it's WIP in !46094 (closed).

{
  "Records": [
    {
      "EventSource": "aws:sns",
      "EventVersion": "1.0",
      "EventSubscriptionArn": "arn:aws:sns:eu-west-1:000000000000:cloudwatch-alarms:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
      "Sns": {
        "Type": "Notification",
        "MessageId": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
        "TopicArn": "arn:aws:sns:eu-west-1:000000000000:cloudwatch-alarms",
        "Subject": "ALARM: \"Example alarm name\" in EU - Ireland",
        "Message": "{\"AlarmName\":\"Example alarm name\",\"AlarmDescription\":\"Example alarm description.\",\"AWSAccountId\":\"000000000000\",\"NewStateValue\":\"ALARM\",\"NewStateReason\":\"Threshold Crossed: 1 datapoint (10.0) was greater than or equal to the threshold (1.0).\",\"StateChangeTime\":\"2017-01-12T16:30:42.236+0000\",\"Region\":\"EU - Ireland\",\"OldStateValue\":\"OK\",\"Trigger\":{\"MetricName\":\"DeliveryErrors\",\"Namespace\":\"ExampleNamespace\",\"Statistic\":\"SUM\",\"Unit\":null,\"Dimensions\":[],\"Period\":300,\"EvaluationPeriods\":1,\"ComparisonOperator\":\"GreaterThanOrEqualToThreshold\",\"Threshold\":1.0}}",
        "Timestamp": "2017-01-12T16:30:42.318Z",
        "SignatureVersion": "1",
        "Signature": "Cg==",
        "SigningCertUrl": "https://sns.eu-west-1.amazonaws.com/SimpleNotificationService-xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx.pem",
        "UnsubscribeUrl": "https://sns.eu-west-1.amazonaws.com/?Action=Unsubscribe&SubscriptionArn=arn:aws:sns:eu-west-1:000000000000:cloudwatch-alarms:xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx",
        "MessageAttributes": {}
      }
    }
  ]
}

Contributes to #262707 (closed).

Please see individual commit messages for more information.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team