xdr: Easily created MuxedAccount objects from a G-address and a memo ID.

[Shaptic]

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

Introduces MuxedAccountFromAccountId, which makes it easy to turn an account ID (G...) into a full MuxedAccount with a memo ID (i.e. representing an M... address). Also introduces MuxedAccount.GetId(), making it easy to extract the underlying memo ID if it's present.

Why

If you have a G-address, it should be easy to create M-addresses from it.

Known limitations

n/a

[bartekn]

It would be super helpful for me when writing tests for #3722. Please merge.