allow tuning the proxies http client

[butonic]

Seeing blocking requests under high load we revisited the http client configuration used by our proxy. Currently, our internal connections use http1.1 by default because backend traffic to other services is not encrypted. That is why we change two defaults:

1. ForceAttemptHTTP2 is now disabled. It just creates connection establishment overhead. When backend traffic to opencloud services is not limited to localhost AND is encrypted this should be enabled.
2. MaxIdleConnsPerHost is now increased from the default 2 to 100 as the proxy otherwise cannot reuse the already established http 1.1 connections.

All other client options that were previously hardcoded can now also be tuned.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 0 duplication

Metric	Results
Duplication	0

View in Codacy

🟢 Coverage 42.11% diff coverage · +0.00% coverage variation

Metric	Results
Coverage variation	✅ +0.00% coverage variation (-1.00%)
Diff coverage	✅ 42.11% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (97908ac)	84010	19254	22.92%
Head commit (faa8746)	84020 (+10)	19257 (+3)	22.92% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#3278)	19	8	42.11%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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