Add custom http destination handler for consolidated audit event destinations

There is 1 validation and 1 helper method pending for custom http destination handler:

1. Validation for secret_token and generating it if not provided, it is a little bit different from aws and gcp as in them, the user necessarily should provide a secret_token but in case of http, secret_token is optional and if not provided, we generate it ourselves. Also, in case of aws and gcp, we do not return secret_token in query api but since for http it is generated by us, so it should be returned in response of query api for the destination. Reference MR: !143914 (merged)
2. Http destinations also allows custom headers to be added for the destination. A method for getting the list of headers need to be added, some work has been done in !165160 (merged).

Verification Steps:

• Ability to create instance and group level external audit event streaming destinations: !147888 (merged)
• Check the validation: !143914 (comment 2162193641)
• Validate headers hash: !143914 (comment 2162193641)

Verify

1. Config is validated per each destination (aws, gcp, http)
2. Secret token is required for gcp and aws, but not http; if it is not passed, one is generated.
3. Secret token, when queried for, is not returned for gcp and aws, and is for http.
4. No two instance destinations can share the same url
5. Two distinct groups can use the same url
6. Secret token for http category is passed automatically by headers, and cannot be overwritten
7. Only 'active' headers are returned