feat: add cache abstraction for fetching signing keys (#14777)

- Adds the database implementation for fetching and caching keys
used for JWT signing. It's been merged into the `keyrotate` pkg and
renamed to `cryptokeys` since they're coupled concepts.

coderd/cryptokeys/keycache.go

@@ -0,0 +1,21 @@
+package cryptokeys
+
+import (
+	"context"
+
+	"golang.org/x/xerrors"
+
+	"github.com/coder/coder/v2/codersdk"
+)
+
+var (
+	ErrKeyNotFound = xerrors.New("key not found")
+	ErrKeyInvalid  = xerrors.New("key is invalid for use")
+	ErrClosed      = xerrors.New("closed")
+)
+
+// Keycache provides an abstraction for fetching signing keys.
+type Keycache interface {
+	Signing(ctx context.Context) (codersdk.CryptoKey, error)
+	Verifying(ctx context.Context, sequence int32) (codersdk.CryptoKey, error)
+}