Provide ability to customize HTTP client in ClientRegistrations and JwtDecoders

[jgrandja]

We should provide the ability to customize the underlying HTTP client in ClientRegistrations and JwtDecoders.

As part of this task, we should also provide the ability to customize/override the default (issuer) validation. Related gh-18992

[evgeniycheban]

Hi @jgrandja, can I work on this?

[jgrandja]

Thank you @evgeniycheban. I've assigned it to you.

The plan is to get this into 7.2.