feat: new external signer to handle external signer provider such as Dfns

[fondation451]

Motivation and Resolution

Before, there was only a default signer with direct access to the private key. I have been using wallet provider such as Dfns which provides signing functions as a service. I derived from the implementation of the default signer, a signer that rely on an external service. I think that it may be useful to have it in the library.

Usage related changes

• Introduction of a new signer to use external signing service such as Dfns

Development related changes

• It is based on the default signer. All the method that needs the private key are overwritten to use the external service

Checklist:

• Performed a self-review of the code
• Rebased to the last commit of the target branch (or merged it into my branch)
• Linked the issues which this PR resolves
• Documented the changes in code (API docs will be generated automatically)
• Updated the tests
• All tests are passing

[tabaktoni]

I don't see a gain in doing a simple wrapper, as any dev can extend Signer on their own for whatever it's needed.
But if we are going to merge this, it would need some examples in the documentation on how to use it.
Can you add these examples @fondation451 for the Dfns you mentioned?