clients/stellarcore: Add support for Core's new (breaking) `/getledgerentry` endpoint

[Shaptic]

PR Checklist

PR Structure

• [x] This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• [x] This PR avoids mixing refactoring changes with feature changes (split into two PRs otherwise).
• [x] 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

• [x] 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 reviewed the changes in this PR and if I consider them worthwhile for being mentioned on release notes then I have updated the relevant CHANGELOG.md within the component folder structure. For example, if I changed horizon, then I updated (services/horizon/CHANGELOG.md. I add a new line item describing the change and reference to this PR. If I don't update a CHANGELOG, I acknowledge this PR's change may not be mentioned in future release notes.
• [ ] 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

Adds support for a new endpoints:

• /getledgerentry
• "hidden" (private method) support for /getinvocationproof
• "hidden" support for /getrestorationproof

Note that these all have the same request format (see #5426 or the PR itself for details), so they share a generic invocation method.

Why

Partially closes #5426.

Known limitations

Needs end-to-end testing, though basic unit tests are present.