feat(config): add shared _parse_headers helper for declarative config exporters

[MikeGoldsmith]

Description

Extracts _parse_headers — which merges an OTLP headers struct and a headers_list string into a single dict — into a new shared _common.py module.

This function is identical across the tracer, meter, and logger provider config PRs (#4985, #4987, #4990). Centralising it here means those PRs can import from _common rather than each defining their own copy, and keeps the logic in one place to test and maintain.

Note: this does not consolidate _parse_headers with _parse_attributes_list in _resource.py — they look similar but differ semantically: resource attributes are URL-decoded (urllib.parse.unquote), headers are not.

Type of change

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

How Has This Been Tested?

11 unit tests added in tests/_configuration/test_common.py covering:

• both inputs None → returns None
• headers_list parsing (single, multiple, whitespace, value-contains-=, malformed)
• struct headers only (including None value → empty string)
• struct headers take priority over headers_list for the same key

Does This PR Require a Contrib Repo Change?

• No.

Checklist:

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

Assisted-by: Claude Sonnet 4.6