[REST Connector] Investigate possible usage of using Request based config

[sbuettner]

What should we do?

Based on the discussion in this PR we should investigate whether we can make use of request based http configuration by leveraging Apache HTTP Clients feature of HttpRequestBase based classes.

Why should we do it?

This would allow to separate the request configuration from the Client configuration and to use and re-use a single Http Client for all outgoing Http requests instead of creating one per request. This could unlock connection handling performance limitations as we as the ability to cache certain data or re-use connections to the same server (only applies to SM as we are bound to the CloudFunction on SaaS)

Additional context

SUPPORT-31940

[johnBgood]

Additional details:

• We would need to create a HttpUriRequestBase and pass it to the builder, instead of using the ClassicRequestBuilder
• Create the client only once, in the HttpService for instance

[crobbins215]

Linked to https://github.com/camunda/product-hub/issues/3050

[vringar]

Needs to be reevaluated in the absence of Cloud Function and introduction of http-base module

[chillleader]

Another request for token caching in SUPPORT-31940

[sbuettner]

@sbuettner We discussed this and agreed with the team that we use a combination of credentials + oauth url for the key of a cache entry.

[sbuettner]

OAuth Token Cache is now being handled in a dedicated issue: #6690

[chillleader]

We already reuse the same instance of HttpClient per connector type