WebClient follow up

[tomas-langer]

Follow up after PR #7255

Required fixes:

• 4.x: HTTP/2 FlowControlTest - disabled 4.x: Fix tests disabled during WebClient redesign #7286
• 4.x: HTTP/2 ClientFlowControlTest - disabled 4.x: Fix tests disabled during WebClient redesign #7286
• 4.x: HTTP/2 HeadersTest - disabled 4.x: Fix tests disabled during WebClient redesign #7286
• 4.x: WebClient - Shared connection cache switch #7301
• 4.x: Http2WebClientTest#multiplexParallelStreamsGet fails intermittently #7307
• 4.x: HTTP/2 Http2ClientTest#testPriorKnowledge #7271
• 4.x: UriPartTest#testPathNotDecoded - disabled #7273
• 4.x: WebClient HTTP 100-Continue #7270
• 4.x: ObserverManager - remove reflection #7272
• 4.x: HelidonFlavor - remove Nima, switch to SE #7337
• 4.x: Naming - rename ws-client to ws-webclient (or align with naming of HTTP/2) - may be done as part of upleveling Nima
• 4.x: System proxy should be default #7274

Tomas will work on these (fixed by #7341):

• 4.x: Http2Stream:362 should use content encoding context and media context from server (check also for client) configuration, now calls .create()
• HttpClientConfigSupport#addHeader(builder, name...) - javadoc has Set default, it should be Add a default, multiple methods (fixed by Prototype builder update #7281 )
• HttpClientConfigBlueprint line 78 - connect timeout javadoc is wrong (eout(java.time.Duration)
• Add readme for HTTP client - how do the protocols get brokered (HTTP based protocols, non-HTTP protocols), SPIs, APIs etc.