Reparametrize ChannelSignerType by SignerProvider.

Reparametrize ChannelSignerType by SignerProvider.

ChannelSignerType is an enum that contains variants of all currently
supported signer types. Given that those signer types are enumerated
as associated types in multiple places, it is prudent to denote one
type as the authority on signer types.

SignerProvider seemed like the best option. Thus, instead of
ChannelSignerType declaring the associated types itself, it simply
uses their definitions from SignerProvider.