[configgrpc, confighttp] Support lists of name/value pairs for headers

[jade-guiton-dd]

Description

This PR introduces:

• A generic MapList[T] type, which is equivalent to []{ Name string, Value T }, but can be unmarshalled from a map[string]T as well. This type is defined in a new internal/maplist package.
• A specialized version exported as configopaque.MapList

configgrpc and confighttp are changed to use this type instead of map[string]configopaque.String for storing header configuration, so as to be consistent with the SDK declarative config (see tracking issue).

Note that while this is relatively elegant, it causes a number of breaking changes. An alternative would be to keep the current map[string]configopaque.String types, and add an Unmarshal method on the surrounding struct: see this PoC.

Link to tracking issue

Fixes #13930

Testing

I added a basic unit test in internal/maplist.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.26%. Comparing base (a646a2e) to head (8ffff95).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #13996      +/-   ##
==========================================
- Coverage   92.27%   92.26%   -0.01%     
==========================================
  Files         656      657       +1     
  Lines       41050    41093      +43     
==========================================
+ Hits        37877    37914      +37     
- Misses       2172     2176       +4     
- Partials     1001     1003       +2     

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[codeboten]

i think this is the right approach 👍🏻

[jade-guiton-dd]

Next steps:

• File a draft PR in contrib with the corresponding changes, so the breakage can be fixed quickly
• Mark this as ready for review
• File a follow-up issue about deprecating and eventually removing the "map" behavior

[jade-guiton-dd]

I pushed the corresponding changes in contrib on my fork here; all in all, about 150 small changes, mostly in tests.

Along the way, I ended up modifying this PR's core API to make it match the native map API better and make the migration easier. These changes have some notable implications:

• MapList[T] is now meant to be used through a pointer, and has the same distinction between "nil" and "empty but not nil" as map
• MapList[T] now has a few additional methods, all of which will be part of configopaque's exported API. Since the module is stable, this API will immediately be set in stone (at least until v2), so I'd like additional scrutiny on it.
• The Get, TryGet, and Set methods have a linear time complexity, vs. the constant time complexity of using map. I doubt there is a significant risk of users adding so many headers that this becomes a problem, but I'm interested in feedback.

I will mark this PR as ready for review, but as a reminder, we can still change course and go for the "backwards-compatible" approach if the benefits of this PR are not deemed to be worth the additional complexity.

[mx-psi]

If we can do without the * that would be better

[mx-psi]

ditto

[mx-psi]

I am going to mark this as draft until it is ready for another review, feel free to mark as ready whenever that happens!

[jade-guiton-dd]

I think I've addressed all the previous comments, and I've filed a corresponding draft PR in contrib (open-telemetry/opentelemetry-collector-contrib#43701) which is almost passing tests, so I'll mark this as ready for another round of review.

[mx-psi]

This looks good to me. Since this introduces a new type on a 1.x module, I want to get at least one more approval before moving forward with this.