services/horizon: Add /liquidity_pools endpoint

[bartekn]

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

Adds implementation of /liquidity_pools endpoint.

Close #3831.

Why

New endpoint for CAP-38.

Known limitations

I based my code on /claimable_balances which implements the cursors of form [ledger_seq]-[id]. I'm not really sure why we are doing this like this because it won't make the paginated view consistent. For simplicity the cursors here are just IDs.

[tamirms]

For simplicity the cursors here are just IDs

yes, I agree. the cursor should just be the id. also because we don't have an index on the last modified ledger column. we only have indexes on id and the reserve assets.