opentelemetry-instrumentation-aiohttp-client: add ability to capture custom headers

[herin049]

Description

This PR adds the ability to capture custom client headers in the opentelemetry-instrumentation-aiohttp-client library. These changes implement #3962 for the aiohttp 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

[JWinermaSplunk]

nitpick: might change get_custom_header_attributes to get_sanatized_custom_header_attributes, but otherwise looks good to me!

[herin049]

nitpick: might change get_custom_header_attributes to get_sanatized_custom_header_attributes, but otherwise looks good to me!

I was considering adding even more context to this function name e.g. making it get_sanitized_custom_header_attributes or get_normalized_custom_header_attributes, but I personally find the shorter version a bit easier to read at a glance, plus documentation wise, I think the doc-string in the PR documents the behavior of the function pretty well.