Add galexie backfill scripts

[chowbao]

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

Adding Galexie backfill scripts

Why

These scripts and README will serve as an example of possible ways to backfill Stellar network data with Galexie.

Known limitations

• These scripts are messy and should not be used without updates and modifications from the user
• These could be temporary scripts as we come up with a better resources/instructions for running Galexie such as the CDP blog series and updating stellar-docs

[chowbao]

I'm not quite sure exactly how much information we want to provide here. I think the goal is more to be an example than the actual recommendation from SDF for running Galexie for backfills.

I'm open to opinions if y'all have any

[chowbao]

Are these scripts too messy?
Like mentioned above they are more examples than actual scripts to be run