Add signals pallet

Resolves #353

Implements code such that:

- 80% of validators (by stake) must be in favor of a signal for the network to
  be
- 80% of networks (by stake) must be in favor of a signal for it to be locked
  in
- After a signal has been locked in for two weeks, the network halts

The intention is to:

1) Not allow validators to unilaterally declare new consensus rules.

No method of declaring new consensus rules is provided by this pallet. Solely a
way to deprecate the current rules, with a signaled for successor. All nodes
must then individually decide whether or not to download and run a new node
which has new rules, and if so, which rules.

2) Not place blobs on chain.

Even if they'd be reproducible, it's just a lot of data to chuck on the
blockchain.

Cargo.lock

@@ -8566,6 +8566,7 @@ dependencies = [
  "serai-coins-pallet",
  "serai-in-instructions-pallet",
  "serai-primitives",
+ "serai-signals-pallet",
  "serai-staking-pallet",
  "serai-validator-sets-pallet",
  "sp-api",
@@ -8584,6 +8585,20 @@ dependencies = [
  "substrate-wasm-builder",
 ]
 
+[[package]]
+name = "serai-signals-pallet"
+version = "0.1.0"
+dependencies = [
+ "frame-support",
+ "frame-system",
+ "parity-scale-codec",
+ "scale-info",
+ "serai-primitives",
+ "serai-validator-sets-pallet",
+ "sp-core",
+ "sp-io",
+]
+
 [[package]]
 name = "serai-staking-pallet"
 version = "0.1.0"