Move common HTTP auth mechanisms (Basic Auth, Bearer Token) to a shared module #6767

Per @kkondaka's feedback on #6743, the Basic Auth and Bearer Token auth in the Prometheus scrape source (ScrapeAuthenticationConfig, BasicAuthenticator, BearerTokenAuthenticator) are generic enough to be reused by other HTTP-based sources and sinks.

We should pull these into a common module so plugins like the HTTP source, HTTP sink (#6747), and Prometheus source can share the same auth code instead of each rolling their own.

This also makes it easier to add new auth schemes (like SigV4) down the road since you'd only implement it once.

Related PR: #6743