sevices/friendbot: Allow friendbot to include cloudflare derived IP address in request logs

[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

Friendbot uses a logging middleware which is used to log incoming requests. Those logs include the client's IP address. However, that ip address is incorrect if the friendbot http server is load balanced by cloudflare. This PR implements a configuration option which enables friendbot to include the real client ip derived from cloudflare in the request logs.

Why

It is useful to see real ips in the logs so we can see observe traffic patterns

Known limitations

[N/A]

[sreuland]

are there any integration tests in the repo that bring up friendbot to confirm this config change works?

[tamirms]

@sreuland I added a unit test. Does that look good to you?