services/horizon/cmd: Check ingest flag before executing horizon db migrate commands

[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

Close #4580

Only apply migrations in the horizon db migrate commands if the ingest flag is true.

Why

The horizon postinstall script in debian runs the db migrate up command

https://github.com/stellar/packages/blob/master/stellar-horizon/debian/stellar-horizon.postinst#L44

However, this command will fail if horizon is configured to use a read only connection (e.g. horizon is configured for request serving but not ingestion). When the command fails the debian installation will also fail.

To fix this issue we check that horizon ingestion is enabled before attempting to execute migrations. If ingestion is not enabled we print a warning and skip the migrations entirely.

Known limitations

Note that no error is returned if ingestion is disabled even though ingestion is a prerequisite for running the db migrate up command. Although this is a bit counter intuitive, it is consistent with the behavior of the --apply-migrations flag #3586