sign transactions using a signer interface instead of a private key

[typestring]

Description

Sign transactions using a signer interface instead of a private key. This will allow to use custom singers such as a remote signer.

Related Issue Or Context

We are experimenting with your repo and the only problem we are facing right now is that one of our requirements is using a remote signer.
With this changes a custom signer could be implemented and passed down to evmclient to sign transactions without the need of a private key.

Closes: #

How Has This Been Tested? Testing details.

provided unit tests and local setup

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Checklist:

• I have commented my code, particularly in hard-to-understand areas.
• I have ensured that all acceptance criteria (or expected behavior) from issue are met
• I have updated the documentation locally and in chainbridge-docs.
• I have added tests to cover my changes.
• I have ensured that all the checks are passing and green, I've signed the CLA bot

[CLAassistant]

All committers have signed the CLA.

[mpetrun5]

Thanks a lot for the contribution.
Looks great!