services/horizon: Improve Captive Stellar-Core validation

[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

This commit improves Captive Stellar-Core flags validation:

• Passing ingestion flags (Captive Stellar-Core or Stellar-Core DB params) without enabling ingestion (--ingest) throws an error. Close Horizon should error if Captive Stellar-Core are passed but ingest=false #3473.
• Allow starting HTTP server without ingestion without having to set ENABLE_CAPTIVE_CORE_INGESTION=false. Close Cannot start non-ingesting Horizon without setting ENABLE_CAPTIVE_CORE_INGESTION=false #3466.

Why

Some users need hints how to configure Captive Stellar-Core.

Known limitations

This change can break some scripts that pass extra config flags without setting --ingest.