txnbuild: Add ToXDR functions which do not return errors

[tamirms]

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

Add ToXDR() functions for Transaction and FeeBumpTransaction instances which return xdr transaction envelopes without errors.

Why

The reason why TxEnvelope() returns an error is that it copies the internal transaction envelope by marshaling and unmarshaling the value. However, if we return a reference to the internal transaction we do not need to handle any error cases. Most users probably will not be modifying the contents of the returned transaction envelope. Having a function which returns a transaction envelope without a possible error is more ergonomic.

Known limitations

[N/A]