Introduce SetCustomReceiver in the logger

[niyatim23]

Summary

SetCustomReceiver configures the ECS Agent logger to use a custom logger implementation. This allows external applications to intercept and handle ECS Agent logs in their own way, such as sending logs to a custom destination or formatting them differently. More details can be found here
The custom receiver must implement the CustomReceiver interface, which requires:

• GetTimestampFormat(): returns the desired timestamp format (e.g., "2006-01-02T15:04:05Z07:00")
• GetOutputFormat(): returns the desired output format ("logfmt", "json", or "windows")
• Log handling methods (Debug, Info, Warn, etc.)

Implementation details

• Introduce a new public API SetCustomReceiver which accepts a customReceiver. This path is independent of the Config and creates its own custom config. It uses this config along with the receiver to create a replacement logger. The formatting of the message is still handled by seelog.
• Introduce locks for timestampFormat to avoid race conditions while accessing it asynchronously

Testing

Tested via the unit tests to make sure that the logging functions in the receiver were triggered with the correct message

New tests cover the changes: Yes

Description for the changelog

Additional Information

Does this PR include breaking model changes? If so, Have you added transformation functions?

No

Does this PR include the addition of new environment variables in the README?

No

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.