Implement CAP 15 (fee bump) support in Go SDK

[ire-and-curses]

This work needs to be done in two parts.

• In part 1, we release an SDK that sends protocol 12 transactions, and can read protocol 13 as far as possible.

• In part 2, we release an SDK that reads and writes protocol 13 transactions.

• Update SDK so that it is compatible with the new horizon transaction response structure

• Update SDK to allow transaction lookup by either its hash or its inner hash (in the case the transaction is a fee bump)

• Consider what other features could make look ups for fee bumps ergonomic

• Update SDK to build transactions from XDR from both protocol 12 and protocol 13, as far as possible. This means shimming for part 1.

• Update SDK with functionality to create and submit fee bump transactions. Note that fee bump transactions have a different fee rate calculation compared to normal transactions.

• Consider SEP 10 implications of fee bump transactions. We probably want to add the restriction that a fee bump transaction cannot be a valid SEP 10 challenge transaction.

Parent issue: #2360

[ire-and-curses]

@tamirms I made a first sketch summarising the work, please edit at will. Also, if it makes more sense to split this issue to address the two different client releases (before and after the vote), please do so.

[bartekn]

Presumably fixed by #2495. Can you confirm? If not, please reopen.