Context improvements explorations

[jponge]

This is a draft of improvements to the Context API in a bid to improve framework integrations, and especially support context scoping rules that seems coherent.

For instance we want to make sure that merging ensures that each upstream branch has its own context rather than a single one:

Uni a
Uni b

a (ctx_1)
 \-join(a, b) -> (root_ctx)
 /
b (ctx_2)

where ctx_1 and ctx_2 are forked from root_ctx at the pipeline subscription time

[jponge]

(this is a work-in-progress, for instance all operators that should fork a context are not doing it as I'm writing this)

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.05%. Comparing base (baef9fb) to head (fa74305).
⚠️ Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
...y/operators/multi/MultiContextForkingOperator.java	83.33%	1 Missing and 1 partial ⚠️
...llrye/mutiny/operators/multi/MultiRetryWhenOp.java	71.42%	1 Missing and 1 partial ⚠️
...tation/src/main/java/io/smallrye/mutiny/Multi.java	66.66%	1 Missing ⚠️
...entation/src/main/java/io/smallrye/mutiny/Uni.java	66.66%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2049      +/-   ##
============================================
- Coverage     89.16%   89.05%   -0.11%     
- Complexity     3121     3128       +7     
============================================
  Files           412      414       +2     
  Lines         13294    13331      +37     
  Branches       1688     1689       +1     
============================================
+ Hits          11853    11872      +19     
- Misses          813      823      +10     
- Partials        628      636       +8     

Files with missing lines	Coverage Δ
...tion/src/main/java/io/smallrye/mutiny/Context.java	91.80% <100.00%> (+0.57%)	⬆️
...va/io/smallrye/mutiny/operators/AbstractMulti.java	100.00% <100.00%> (ø)
...java/io/smallrye/mutiny/operators/AbstractUni.java	90.32% <100.00%> (+0.32%)	⬆️
...allrye/mutiny/operators/uni/UniAndCombination.java	90.65% <100.00%> (ø)
...utiny/operators/uni/UniContextForkingOperator.java	100.00% <100.00%> (ø)
...mallrye/mutiny/operators/uni/UniOrCombination.java	91.17% <100.00%> (ø)
...lrye/mutiny/operators/uni/builders/UniJoinAll.java	95.83% <100.00%> (ø)
...ye/mutiny/operators/uni/builders/UniJoinFirst.java	89.39% <100.00%> (ø)
...tation/src/main/java/io/smallrye/mutiny/Multi.java	86.36% <66.66%> (-3.12%)	⬇️
...entation/src/main/java/io/smallrye/mutiny/Uni.java	88.88% <66.66%> (-2.03%)	⬇️
... and 2 more

... and 13 files with indirect coverage changes

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[FroMage]

I'd probably be able to give better feedback if this proposal had docs and sample usage, but perhaps we should first establish better base for docs as discussed at #2037 ?

[jponge]

This one is not even ready for review 😉