opamp-client: support text effective config bodies

[pmcollins]

Description

Adds support for building OpAMP effective config messages from non-JSON text or bytes bodies. This allows callers to report effective configuration with MIME types such as text/plain, while preserving the existing application/json behavior.

This aligns with the OpAMP spec's AgentConfigFile model, where body is raw bytes and content_type describes those bytes.

Addresses #4535

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

Added OpAMP unit tests.

• Added coverage for text/plain effective config bodies
• Added coverage for bytes effective config bodies
• Added coverage that full-state OpAMP messages preserve content_type and body

Does This PR Require a Core Repo Change?

• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[JWinermaSplunk]

Did we want to add coverage for a non serializable value with content type application/json?

[pmcollins]

Did we want to add coverage for a non serializable value with content type application/json?

Sure, done.

[emdneto]

Thanks for the PR!

Just a heads-up: we no longer update CHANGELOG.md directly. The changelog is now generated from changelog fragments using Towncrier.

Please add the appropriate changelog fragment for this change instead of editing CHANGELOG.md manually. You can find the instructions and expected format in CONTRIBUTING.md.