recoverysigner: make allowed-source-account optional

[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 makes the --allowed-source-account flag optional.

Why

When ALLOWED_SOURCE_ACCOUNT is not specified or is empty, strings.Split(opts.AllowedSourceAccounts, ",") will return a slice containing an empty string. And we will get an error at keypair.ParseAddress(addressStr).

Known limitations

N/A

[leighmcculloch]

The change looks good. One question.