txnbuild: Use xdr.Price to represent prices in txnbuild instead of strings.

[tamirms]

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

Fixes #2514

Use xdr.Price to represent prices in txnbuild instead of strings.

Why

It is impossible to represent prices as strings without finite decimal expansion with a reasonable number of decimal places.

Known limitations

[N/A]

[leighmcculloch]

👏🏻 🎉

[bartekn]

LGTM! I wonder if we should create txnbuild.Price struct instead of using xdr.Price.