Introduce a Transaction type that supports higher fees beyond the uint32 MAX VALUE

[anupsdf]

Why?

In Stellar-transaction.x XDR, we have Transaction::fee defined as uint32 type. With introduction of Soroban, the total fees could go higher in future. The resource usage fees, rent fees and inclusion fees could add up to go beyond uint32 MAX VALUE.

What?

We could add a new Transaction type that can take i64 fee.