Update SignersProcessor to use FastBatchInsertBuilder

[urvisavla]

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

Optimize the SignersProcessor to use FastBatchInsertBuilder which uses COPY to bulk insert into the database table account_signers.

• Using the FastBatchInsertBuilder when inserting into the account_signers table during building state (i.e. useLedgerEntryCache is false).
• Introducing another optimization in the insertion process during live ingestion (i.e., useLedgerEntryCache is true) to also use bulk insert via FastBatchInsertBuilder.
• Refactor the code to reduce code duplication and improve overall readability.

Why

see #5098

Known limitations

Previously in SignersProcessor, during live ingestion, each row was inserted individually and a ingest.StateError was thrown in case of failure. With the bulk write, this individual check is no longer possible and so the system now throws an error for the entire batch if a failure occurs.