Added support of Bearer Auth for HTTP Exporters

[pavankrish123]

Current implementation for Bearer token auth only supports gRPC clients. This implementation enables the functionality for HTTP clients as well.

Link to tracking Issue: #5927

Testing: : Unit tests and verfied on the mock server Auth headers were set properly.

Documentation: Updated README.md with new path as well.

[pavankrish123]

@jpkrohling @anuraaga please review - thank you.

[jpkrohling]

LGTM, but I just realized we don't have support for auth in the confighttp struct like we have for configgrpc, so, we cannot have an HTTP receiver that requires auth.

[pavankrish123]

Thanks @jpkrohling - shall I open an issue to handle receivers as well for consistency or we don't want to tackle it?

This pr brings consistency on exporters side though.

BTW, not sure what's happening with k8cluster unit test

[jpkrohling]

Created #5973 to track the test failure on unrelated code, and tests restarted.

[jpkrohling]

Another unrelated test failure. Created #5983, tests restarted.

[pavankrish123]

Thank you - Looks like tests are fine now.

[jpkrohling]

I don't seem to have merge powers yet, but this is ready to be merged.

[bogdandrutu]

@jpkrohling fixed the power 👯