txnbuild: Add client domain support for sep 10

[OwenJacob]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've reviewed the changes in this PR and if I consider them worthwhile for being mentioned on release notes then I have updated the relevant CHANGELOG.md within the component folder structure. For example, if I changed horizon, then I updated (services/horizon/CHANGELOG.md. I add a new line item describing the change and reference to this PR. If I don't update a CHANGELOG, I acknowledge this PR's change may not be mentioned in future release notes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Adds support for client domains in build and read challenge transactions

Why

Closes a long outstanding issue #3780 and aligns the Go txnbuild package with SEP-10

Known limitations

I decided not to break the existing API 😅

[Copilot]

Pull Request Overview

This PR adds support for client domains in SEP-10 challenge transactions by introducing new functions BuildChallengeTxWithClientDomain and ReadChallengeTxWithClientDomain. This enhancement allows clients to specify a domain they control during the authentication flow, improving security and alignment with the SEP-10 specification.

Key changes:

• Added BuildChallengeTxWithClientDomain function that extends challenge transaction building with optional client domain support
• Added ReadChallengeTxWithClientDomain function that validates client domain operations during challenge verification
• Refactored existing functions to use the new implementations while maintaining backward compatibility

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
txnbuild/transaction.go	Implements core client domain functionality with new build and read functions
txnbuild/transaction_test.go	Adds comprehensive test coverage for client domain scenarios including validation and error cases

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