in_http plugin: supporting http auth

**Is your feature request related to a problem? Please describe.**

This feature request is derived from the Fluentd Mailing list question.

* [http auth for in_http plugin](https://groups.google.com/g/fluentd/c/5dsZR4NCUII)

fluentd supports Basic Auth by http output plugin,

https://docs.fluentd.org/output/http#less-than-auth-greater-than-section

but it seems that http input plugin doesn't support Basic Auth.
Because of this inconsistency, it can't send directly between two fluentd instances.

**Describe the solution you'd like**

Implement Basic Auth support in input http plugin side.

**Describe alternatives you've considered**

N/A

Or give up to use Basic Auth and use an alternative method (TLS encryption)

**Additional context**

N/A