Introduce ChannelSignerType.

Rather than using a holder_signer of a specific
signer type in Channel and ChannelContext, this
allows us to hold an enum such that depending on
the type of channel, the appropriate signer could
be held in its respective variant.

Doing so required the reparametrization of Channel
from using a Signer to using the SignerProvider
trait. This percolated down to the ChannelManager
and multiple tests.

Now, when accessign various signer methods, there
is a distinction between accessing methods defined
for all signers on ChannelSigner, and accessing
type-specific methods using accessors such as
`as_ecdsa`.

Author: arik-so