Add Extensions field to Go SettleResponse for parity with TypeScript SDK

[javierpmateos]

Description

The TypeScript SDK's SettlementResponse includes an extensions field (Record<string, unknown>) that enables extension data like the recently merged offer-receipt (PR #935). The Go SDK's SettleResponse struct does not have this field.

This PR adds Extensions map[string]interface{} with json:"extensions,omitempty" to the Go SettleResponse struct, bringing it to parity with the TypeScript SDK.

This is a non-breaking change — the field is omitempty, so existing serialization is unaffected. No existing tests reference the absent field.

Related: #1802

Tests

cd go && go test ./...

All 22 test packages pass (tested locally on Go 1.22). The new field is omitempty and not referenced by any existing test, so no test changes are needed.

Checklist

• I have formatted and linted my code
• All new and existing tests pass
• My commits are signed (required for merge)
• I added a changelog fragment for user-facing changes

[cb-heimdall]

🟡 Heimdall Review Status

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[vercel]

@javierpmateos is attempting to deploy a commit to the Coinbase Team on Vercel.

A member of the Team first needs to authorize it.

[phdargen]

Thanks for the contribution @javierpmateos!

Please rebase against main and resolve the merge conflict.
Please also add the extensions field to VerifyResponse analog to the TS implementation

[phdargen]

We require commit signing, please rebase and verify your commits.
See here for instructions: https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification

[javierpmateos]

@phdargen Done .... rebased against main, added Extensions to VerifyResponse as well, and signed the commit. Ready for review.