node/network/bridge: Define protocol names as str

[mxinden]

Companion for paritytech/substrate#6967.

Notification protocol names are in practice always valid utf8 strings.
Instead of treating them as such in the type system, thus far they were
casted to a [u8] at creation time.

With this commit protocol names are instead treated as valid utf8
strings throughout the codebase and passed as Cow<'static, str>
instead of Cow<'static, [u8]>. Among other things this eliminates the
need for string casting when logging.

As far as I can tell none of these protocol names are used anywhere.
I still think it is worth keeping them in sync.

[ghost]

Waiting for commit status.