opentelemetry-instrumentation-requests: add ability to capture custom headers

[herin049]

Description

This PR adds constants for two new environment variables:

• OTEL_INSTRUMENTATION_HTTP_CAPTURE_HEADERS_CLIENT_REQUEST
• OTEL_INSTRUMENTATION_HTTP_CAPTURE_HEADERS_CLIENT_RESPONSE

to the opentelemetry-util-http library and adds the ability to capture custom client headers in the opentelemetry-instrumentation-requests library. These changes implement #3962 for the requests HTTP client library.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Basic set of unit tests have been added.

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• 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