ledgerexporter: Add lexicographical ordering token to filenames

[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

For HUBBLE-395
Add a lexicographical ordering token as a prefix to the partition and file names so that the first file within a cloud storage bucket (e.g. GCS) is the largest ledger sequence number.

Example: /bucket/FFFF05FE--64001-128000/FFFF05FE--64001-64064.xdr.gz
Note that the sort token is MaxUint32-(start partition ledger or start file ledger) in hex

Why

The current ordering of partitions and files are integers but the sorting in GCS is alphanumeric making some files appear out of order within GCS.

Known limitations

This does add more text to the filenames which make it slightly less human readable.

[tamirms]

looks good! I left a couple of minor suggestions for your consideration