SignerV2 interface is confusing

[samlaf]

The functions in the signerv2 (for eg this) return bind.SignerFn instead of SignerFn, which is confusing because the txMgr needs a SignerFn. So the user either has to wrap the bind.SignerFn themselves, or use the more complicated (and also confusing) signerFromConfig. This one is confusing because forcing user to use the config which has too many fields and unclear which ones are mandatory and which ones are not.

I think we should just make the simple constructors return SignerFn directly, and then the SignerFromConfig can just call those and return directly, instead of wrapping their returned bind.SignerFn in a SignerFn.

[samlaf]

@shrimalmadhur what's the state of this? Also why do we still have signer around? Is that just for backwards compatibility? Is it still relevant to anything/anyone?