[dynamic control] Add SourceKind as sources are a known list

[jackshirazi]

Description:

SourceKind enum added, the OTEP mentions explicit sources, and though this could widen, it will still likely be an enum. Making it an enum allows consistent handling of different sources

Existing Issue(s):

#2546

Testing:

Added

Documentation:

Added

Outstanding items:

The SourceKind is not yet applied. The next PR will start applying it, this PR is kept smaller for easier review

[Copilot]

Pull request overview

Adds a SourceKind enum to represent the known set of dynamic-control policy configuration sources (file / OpAMP / HTTP / custom), aligning with the OTEP’s notion of explicit sources and enabling consistent handling as the feature set evolves.

Changes:

• Introduce SourceKind enum with stable lowercase config values and a parsing helper (fromConfigValue).
• Add unit tests covering config value stability, case-insensitive parsing, whitespace trimming, and unknown/null handling.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dynamic-control/src/main/java/io/opentelemetry/contrib/dynamic/policy/source/SourceKind.java	Adds SourceKind enum + config value normalization/parsing API.
dynamic-control/src/test/java/io/opentelemetry/contrib/dynamic/policy/source/SourceKindTest.java	Adds tests validating SourceKind config value and parsing behavior.