Invoke contract from RPC server

[2opremio]

What

Allow invoking a contract through a remote RPC server (i.e. from a real network) instead of local sandbox

Note that, as getContractData is yet to be implemented by the RPC server, it is allowed to provide the contract through the --wasm flag. We should probably remove this in the future.

Also:

• refactor transaction signing

Known limitations

• It doesn't yet print the costs or the invocation results (let's do that later since I want to unblock end to end testing)
• this requires exp/services/soroban-rpc: Add simulate transaction endpoint go#4621

[2opremio]

CC @sreuland

[tamirms]

@2opremio FYI because latestLedger is an int64 value it will be represented as a String in the json response

[sreuland]

@2opremio @leighmcculloch , in terms of using for e2e tests, what would the cli invocation look like for hello world:

the contract fn prototype:
pub fn hello(env: Env, to: Symbol) -> Vec<Symbol>

assuing cli invocation would look something like:

soroban invoke --id <contract_id> \
  --account GBZXN7PIRZGNMHGA7MUUUF4GWPY5AYPV6LY4UV2GL6VJGIQRXFDNMADI
  --fn hello \
  --arg "world" \