horizonclient: add support for join param.

[poliha]

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

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.

Summary

Allow users to get transaction information when querying an operation endpoint.
Closes #1521

Goal and scope

Add the join parameter to operations and payments request. This allows horizon server to return transaction information along with an operation response

Summary of changes

• Added join param type
• Updated OperationRequest struct
• Added tests to check that the URL is built correctly.

Known limitations & issues

What shouldn't be reviewed