[feature] Basic Auth handler

[stokito]

How about to add a basic auth handler? This is still a good option for 90% of APIs.
Unfortunately the golang doesn't provide it out of the box.
This leads to problems because developers starts to write their own which may not be not secure and fast.
Or devs may start to use some auth library which may be configured not properly and again become vulnerable.
Even while there may be a separate libraries for this I think it may be good option to have it in the library so users will have it with less dependencies.

It's questionable how feature rich it should be. I think it must be plain simple and focused on performance. Just as a starter but also because other more feature rich (and slow) libraries exists.
E.g. no password hashing: for API credentials this not needed because you can simply reset credentials.

Alternatives:

• Here I wrote myself but I don't like it anymore for few minor reasons: https://github.com/stokito/go-http-server-basic-auth/blob/master/auth_handler.go
• Most popular library https://github.com/abbot/go-http-auth
• Some popular gist that many users just copy and paste https://gist.github.com/elithrar/9146306 It's mentioned in the first googled StackOverflow topic :) It's written by the @elithrar
• Many others libs but none seems good to me https://pkg.go.dev/search?q=basic+auth
• One of them looks easy to use https://github.com/99designs/basicauth-go/blob/2a93ba0f464d/basicauth.go
• Prometheus also has basic auth handler with BCrypt support https://github.com/prometheus/exporter-toolkit/blob/master/web/handler.go#L105

If you think that it may be useful to add the basic auth handler then I may rework my version to be more useful but still easy to use and send a PR.