txnbuild: adds a 5 minute grace period to ReadChallengeTx MinTime constraint

[JakeUrban]

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

Adds a 5 minute grace period to ReadChallengeTx's Transaction.Timebounds().MinTime constraint to account for clock drift between the client and server.

Why

The JS SDK recently merged a similar change (commit), which is now requested for the Go SDK (issue).

Known limitations

N/A

[JakeUrban]

I had originally created my feature branch on master and force-pushed after rebasing on the 8.0 release branch.

[JakeUrban]

@ire-and-curses I'm not sure who should review this so feel free to add someone and remove yourself 👍

[leighmcculloch]

I think this is good. 👏🏻 We could also make it configurable, but there's no rush to do that now and we can always expose the config later if needed. There's value in making this configurable in SDKs that are used with clients and servers, but less so with servers.

[leighmcculloch]

@JakeUrban This change is not breaking and you could have it target master, unless this needs to be in a major release for some reason not mentioned here.

[JakeUrban]

I force pushed to rebase on master, no changes to the code though.