feat: change TransactionAuthenticator's get_signature to be async

[tomyrd]

Stemming from this comment. We should change the authenticator to allow async signature requests. This could be done by changing the trait to async or to maybe_async if we want to allow both options.

[varun-doshi]

I'd like to take this

[bobbinth]

Thank you @varun-doshi! But this is actually not a very straight-forward task which may require refactoring in the VM or other places. The main issue is that we can't easily move from sync to async context and back, especially in web assembly. There is one potential approach for how to handle this described in #401 (comment).

[varun-doshi]

Ahh I see
In that case, I can close the PR or if someone wants to work off of the PR? (although there isn't much code change)