Remove InProcessTransactionExecutor and add SignTransaction to the ISigner

[kantagara]

Summary InProcessTransactionExecutor violates a concept where transaction executor knows something about the private key and is actively using it to sign messages. Also, only ISigner himself should know anything about private key.

Suggested features or ways to address the summary

• [ ] Remove InProcessTransactionExecutor completely
• [ ] Handle signing transactions in the ISigner itself, transactionexecutor can only get the signed transaction but he should not sign it himself.

Motivation Our SDK has a consistent architecture

Acceptance Criteria

• [ ] InProcessTransactionExecutor is removed
• [ ] ISigner has SignTransaction method