services/horizon: expose the Latest-Ledger header to browser web pages

[leighmcculloch]

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

Expose the Latest-Ledger header to browser web pages.

Why

It's not possible for web applications that are loading data directly from Horizon to know what ledger the response is based on because response headers are not accessible to a browser page unless the server explicitly lists the response header as being exposed.

Close #3993

Known limitations

N/A

[bartekn]

Good catch!

[leighmcculloch]

@bartekn Could I get this into the next Horizon release?