feat: Add support for excluding ASGI receive/send spans via environment variable in FastAPI

[deepakdinesh1123]

Description

Added the enviornment variable OTEL_PYTHON_FASTAPI_EXCLUDE_SPANS to support excluding the send and receive spans that the ASGI middleware generates.

Fixes #3992

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?

• Disable send spans and check that send spans are ignored
• Disable receive spans and check that receive spans are ignore
• Disable both send and receive spans and check that both are ignored
• Exception raised when invalid values are passed to environment variable
• Check that exclude_spans argument takes priority over the environment variable

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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: deepakdinesh1123 / name: deepakdinesh1123 (f672f37)