services/horizon/internal/actions: Update payload for "Accounts For Signers" end-point.

[abuiles]

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.

Summary

Account for signers was using a partial representation of accounts since we didn't have all the necessary information in the database to build the full representation. Since #1819 we are now ingesting account data and can return all the fields related with an account. This PR updates the end-point making it consistent with the results when fetching a single account or filtering accounts by a given trustline. Fixes #1875

Goal and scope

• Update payload in account for signers.

Summary of changes

• Add new method to history, so we can fetch account entries from a signer.

Known limitations & issues

What shouldn't be reviewed