xdr: Update XDR to meet core's fees changes

[tsachiherman]

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

Update XDR to meet core's fees changes

Why

We need to compile against core.

Known limitations

N/A

[sreuland]

@tsachiherman , I think pr is ready to merge, i had to disable the tests in sac_test.go as they all depended on the preflight http call to core which is gone now, the tests can be re-enabled on later iteration when time to decide how to derive the footprint inline here.

there are working contract tx sub tests in invokehostfunction_test.go which derive the needed footprint for those that can be used as reference, but those are non-sac, so the footprint may not be same approach.

[tsachiherman]

looks good. Unfortunately, since I'm the one who created the PR, I'm unable to approve it.