txnbuild: Allow fee bump transactions to bump v0 transactions

[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

Currently when you try to create a fee bump transaction from a v0 transaction we return an error. This PR does not return an error and instead converts the v0 transaction to a v1 transaction before wrapping it with a fee bump transaction.

Why

Consider the use case of a pre-signed transaction that was made in the distant past. You created the transaction as part of a smart contract, and now you want to execute it, but you need it to have a higher fee than you set back then.

Known limitations

[N/A]

[abuiles]

Looks good! Shall we implement the same across other SDKs too?

[tamirms]

Looks good! Shall we implement the same across other SDKs too?

@abuiles yes, we'll need to implement two changes in the other SDKs

1. stop creating v0 transaction envelopes and instead only create v1 envelopes txnbuild: Make txnbuild create V1 transactions #2640
2. convert v0 transactions to v1 transactions when creating fee bump transactions (this PR)