Implemented getRawTransactionWithSignatures and combineSignatures

[jimni1222]

Proposed changes

This PR introduces getRawTransactionWithSignatures and combineSignatures functions.

1. getRawTransactionWithSignatures
   : This function is a change from the existing signTransactionWithSignature function.
   Creates an RLP encoded transaction, taking as a parameter a transaction object containing signatures or feePayerSignatures.

2. combineSignatures
   : The combineSignatures function combines multiple RLP encoded transactions into a single RLP encoded transaction.

Types of changes

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• Bugfix
• New feature or enhancement
• Others

Checklist

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• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
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• Any dependent changes have been merged and published in downstream modules

Related issues

refer #117

Further comments

Remain PR contents

1. sendSignedTransaction
2. test codes