fix: Implement gogoproto customtype to secp256r1 keys (backport #20027)

[mergify]

Description

These 2 types are being used as customtypes in the proto files, but to do it they need to implement this specific interface.

Closes: #19956

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Author Checklist

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please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• reviewed "Files changed" and left comments if necessary
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• updated the relevant documentation or specification, including comments for documenting Go code
• confirmed all CI checks have passed

Reviewers Checklist

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I have...

• confirmed the correct type prefix in the PR title
• confirmed all author checklist items have been addressed
• reviewed state machine logic, API design and naming, documentation is accurate, tests and test coverage

Summary by CodeRabbit

• New Features
  • Enhanced JSON support for private and public keys, including marshaling and unmarshaling capabilities.
• Tests
  • Added tests for JSON functionality in key handling.

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This is an automatic backport of pull request #20027 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 037cf98 has failed:

On branch mergify/bp/release/v0.47.x/pr-20027
Your branch is up to date with 'origin/release/v0.47.x'.

You are currently cherry-picking commit 037cf98f7.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   crypto/keys/secp256r1/privkey.go
	modified:   crypto/keys/secp256r1/privkey_internal_test.go
	modified:   crypto/keys/secp256r1/pubkey_internal_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md
	both modified:   crypto/keys/secp256r1/pubkey.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally