Add custom mapping related columns to http_integrations

What does this MR do?

Contributes to #262707 (closed)

Adds payload_example and payload_attribute_mapping columns to alert_management_http_integration table.

The columns are intended for storing the custom mapping related information per HTTP integration for alerts.

The payload_example is going to contain an example of an alert payload which is going to be sent to the alert's endpoint of a particular HTTP Integration. It should be similar (or close) to the payload we store in the alert_management_alerts.payload. It is a custom field populated by a user with maintainer (and above) permissions. The HTTP Integrations feature allows having one HTTP Integrations for licenses below GitLab Premium and several for licenses above GitLab Premium.

It can contain a lengthy JSON, but we are going to limit it to 1 MByte.

The payload_attribute_mapping column is going to contain a mapping between an alert attributes we want to display on an alert's page and their paths in the alert's payload.

Can have the following structure:

payload_example:

{
  "alert": {
    "name": "Example",
    "desc": "An alert example",
  },
  "created_at": "2019-12-14T14:15:16Z"
}

payload_attribute_mapping:

{
  "title": { "type": "string", "path": ["alert", "name"] },
  "description": { "type": "string", "path": ["alert", "desc"] },
  "start_time": { "type": "datetime", "path": ["created_at"] }
}

We are not going to search by payload_example, nor by payload_attributes_mapping, thus there is no need to have indexes for these columns.

Migration output

→ be rake db:migrate
== 20201214113729 AddCustomMappingColumnsToHttpIntegrations: migrating ========
-- add_column(:alert_management_http_integrations, :payload_example, :jsonb, {:null=>false, :default=>{}})
   -> 0.0050s
-- add_column(:alert_management_http_integrations, :payload_attribute_mapping, :jsonb, {:null=>false, :default=>{}})
   -> 0.0006s
== 20201214113729 AddCustomMappingColumnsToHttpIntegrations: migrated (0.0058s)

→ be rake db:rollback
== 20201214113729 AddCustomMappingColumnsToHttpIntegrations: reverting ========
-- remove_column(:alert_management_http_integrations, :payload_attribute_mapping, :jsonb, {:null=>false, :default=>{}})
   -> 0.0023s
-- remove_column(:alert_management_http_integrations, :payload_example, :jsonb, {:null=>false, :default=>{}})
   -> 0.0003s
== 20201214113729 AddCustomMappingColumnsToHttpIntegrations: reverted (0.0063s)

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

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• [-] 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
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