add a thread safe context that can be used downstream and mutated

[rubensayshi]

It's currently unsafe to use c.Request.Context from the *gin.Context to pass to downstream functions AND do any mutations to the c.Request.Context.
Even when you do c.Request = c.Request.WithContext(...) you'll end up with the go race detector flagging the unsafe reading of the c.Request and writing to c.Request.

And even when you're not really doing anything .WithContext there's a chance that the context pool causes the race detector to get angry

However it would be really nice to be able to put values into the *gin.Context with context.WithValue() in a thread-safe way that allows downstream usage of *gin.Context as context.Context to access those values.
For example, an auth middleware adding some debugging /loggin related values with context.WithValue() will not be lost along the way, or adding a otel.Tracer to the context with additional options etc.

Making all the places where gin touches c.Request thread-safe with a lock would be a significant undertaking and would have an impact on performance (probably talking about nano seconds, but still non zero impact).

With the MR we're adding a thread-safe way to carry around and mutate a context.Context inside *gin.Context without having to protect c.Request.

In practice the c.Request.Context isn't going to be very interesting to wrap anyway, so loosing that as the "root" context probably isn't a issue for anyone.
It's trivial to make the root InternalContext the c.Request.Context when we do .reset() (if ContextWithFallback is enabled), but I wanted to keep this MR as small as possible.
And it would have one quirky downside that if someone uses UseInternalContext and does something like c.Request = c.Request.WithContext(...) then it wouldn't have any effect.

There's a couple of mentions of issues around this before:

• gin.Context data race in most trivial situation #4117
• gin detects DATA RACE with Go1.21, Go1.22 #3931
• Data Race into context when you don't call func (c *Context) Copy() *Context #1118

[appleboy]

@rubensayshi Please update the PR to the latest source code for the CI/CD flow.

[codecov]

Codecov Report

Attention: Patch coverage is 83.92857% with 9 lines in your changes missing coverage. Please review.

Project coverage is 98.75%. Comparing base (3dc1cd6) to head (cd8ad99).
Report is 99 commits behind head on master.

Files with missing lines	Patch %	Lines
context.go	82.69%	8 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4172      +/-   ##
==========================================
- Coverage   99.21%   98.75%   -0.47%     
==========================================
  Files          42       44       +2     
  Lines        3182     3445     +263     
==========================================
+ Hits         3157     3402     +245     
- Misses         17       32      +15     
- Partials        8       11       +3     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags "sonic avx"	98.74% <83.92%> (?)
-tags go_json	98.74% <83.92%> (?)
-tags nomsgpack	98.73% <83.92%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.23	98.75% <83.92%> (?)
go-1.24	98.75% <83.92%> (?)
macos-latest	98.75% <83.92%> (-0.47%)	⬇️
ubuntu-latest	98.75% <83.92%> (-0.47%)	⬇️

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[rubensayshi]

happy to increase the coverage if there's an actual chance this would get merged! :D