Create CI workflow for ledger exporter release

[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

Two new git CI workflows for the ledger exporter:

1. PR Workflow: Runs whenever a pull request is merged to the master branch and does the following:

• Builds the ledger exporter docker image.
• Tests the image using a fake-gcs-storage emulator.

2. Release Workflow: Runs whenever a release tag is created for the ledger exporter and does the following:

• Extracts the version number from the branch name (e.g., "ledgerexporter-v1.2").
• Builds a docker image tagged with stellar/ledger-exporter:<version_number> (e.g., "stellar/ledger-exporter:1.2").
• Pushes the built image to the Stellar docker repo.

Why

Fixes HUBBLE-492

Known limitations

N/A