xdr: Add AbsBeforeEpoch to ClaimableBalance:Predicate API resource

[sreuland]

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

added a new optional field AbsBeforeEpoch to the API resource ClaimableBalance:Predicate, it is the Unix Epoch value represented by the same custom extended ISO date value in the AbsBefore field.

Why

this extra info for Epoch will enable SDK's to use the epoch for absolute before value of claimable balance predicate rather than attempt to parse the ISO, which gets complicated when the date very large, beyond the ISO standard range of year 9999. Some SDK's like the java SDK have bug fix pending on this change, like Java SDK #354

Closes #4095

Known limitations

[eliane345]

Desculpe por transtornos causados

[eliane345]

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