tests: 🔑 add keyring interfaces to TestNode<C>

[cratelyn]

see #3588. see #3902 (comment).

in order to test the staking and governance components however, we will need
to provide a keyring, so that the engine can sign blocks.

type Keyring = BTreeMap<VerificationKey, SigningKey>;

impl<C> TestNode<C> {
    pub fn keyring(&self) -> &Keyring { todo!() }
    pub fn keyring_mut(&self) -> &mut Keyring { todo!() }
}

this should be in service to tests of the staking component that:

1. create the validator definition for the new validator, generating identity
   and consensus keys
2. insert the consensus signing key into the engine’s keyring
3. simulate a tx that posts that validator definition
4. when the app reports the new validator to the consensus engine, the engine
   can look up its key in the keyring and start using it for signing blocks

[cratelyn]

#3902 added a keyring interface. #4039 removes that, in favor of directly presenting a BTreeMap.

i am going to close this. we'll work on addressing #3995 directly, rather than trying to proactively design interfaces for those tests.