txnbuild: add AddSignatureBase64 function

[howardtw]

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 updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• 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

This PR adds AddSignatureBase64 function to both Transaction object and FeeBumpTransaction object.

Why

In JS SDK, there is addSignature function to add a base64 signature to a transaction. However, there is nothing like this in Go SDK.

In SEP 30, the response of the sign endpoint contains a signature in base64. It would be helpful if there is a function in txnbuild that adds a signature in base64 to the transaction.

Known limitations

N/A

Close #2440

[ire-and-curses]

@howardtw Sorry, I saw this too late. Can you update the txnbuild changelog, unreleased section, to include this?

[howardtw]

@ire-and-curses #2590