feat: ✨ add cross-nodes context propagator to remoting

[Tochemey]

This PR adds first-class support for propagating context values across remoting calls:

• Introduced a context_propagator option on remoting config so outbound RemoteAsk/RemoteTell (actors and grains) inject caller context into headers, and inbound handlers extract those values back into the message context.

• Added a simple header-based propagator and race-safe echo actor to verify propagation end-to-end in tests (ask/tell for both actors and grains).

• Ensured remoting handler test servers use h2c to match the HTTP/2 client and avoid protocol errors.

[Tochemey]

@sbulman The current design only supports context propagation within a single process. After this PR, context values can be propagated across nodes all the way down to the message handlers; the only additional step is implementing remote.ContextPropagator.

[codecov]

Codecov Report

❌ Patch coverage is 93.90244% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.43%. Comparing base (76849e2) to head (63bab33).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
actor/grain_engine.go	75.00%	2 Missing and 2 partials ⚠️
remote/remoting.go	97.67%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #996      +/-   ##
==========================================
+ Coverage   94.22%   94.43%   +0.20%     
==========================================
  Files         139      139              
  Lines       10999    11070      +71     
==========================================
+ Hits        10364    10454      +90     
+ Misses        414      399      -15     
+ Partials      221      217       -4     

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