fix(binding): add explicit interface types in binding_nomsgpack.go

[GLEF1X]

This PR adds explicit interface types to variable declarations in binding_nomsgpack.go to allow third-party libraries to reassign bindings with custom implementations. binding.go has explicit interface types.

Related Issue

Without explicit interface types, variables like binding.JSON are typed as their concrete struct (e.g., jsonBinding) rather than the BindingBody interface. This prevents libraries from reassigning these variables with wrapper types that implement the same interface. Source.
This allows to fix DataDog/dd-trace-go#4114 where the missing explicit types prevent compilation when using the nomsgpack build tag.

Pull Request Checklist

Please ensure your pull request meets the following requirements:

• Open your pull request against the master branch.
• All tests pass in available continuous integration systems (e.g., GitHub Actions).
• Tests are added or modified as needed to cover code changes.
• If the pull request introduces a new feature, the feature is documented in the docs/doc.md.

Thank you for contributing!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.06%. Comparing base (3dc1cd6) to head (1fb09e9).
⚠️ Report is 226 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4478      +/-   ##
==========================================
- Coverage   99.21%   99.06%   -0.16%     
==========================================
  Files          42       44       +2     
  Lines        3182     2986     -196     
==========================================
- Hits         3157     2958     -199     
- Misses         17       18       +1     
- Partials        8       10       +2     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.98% <ø> (?)
-tags go_json	98.92% <ø> (?)
-tags nomsgpack	98.98% <ø> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.99% <ø> (?)
go-1.25	99.06% <ø> (?)
macos-latest	99.06% <ø> (-0.16%)	⬇️
ubuntu-latest	98.99% <ø> (-0.22%)	⬇️

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