Modified signTransaction logic and implemented feePayerSignTransaction

[jimni1222]

Proposed changes

This PR introduces modification signing logic and newly implemented functions.

1. signTransaction is modified for handling RLP encoded raw transaction and appending.

2. feePayerSignTransaction is implemented for fee payer.

3. signTransaction and feePayerSignTransaction function remove duplication of signatures.

Types of changes

Please put an x in the boxes related to your change.

• Bugfix
• New feature or enhancement
• Others

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• I have read the CONTRIBUTING GUIDELINES doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules

Related issues

refer #117

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...