extract select by signer from KeychainProvider

OpenKeychain/src/main/sqldelight/org/sufficientlysecure/keychain/KeySignatures.sq

@@ -0,0 +1,7 @@
+CREATE TABLE IF NOT EXISTS key_signatures (
+    master_key_id INTEGER NOT NULL,
+    signer_key_id INTEGER NOT NULL,
+    PRIMARY KEY(master_key_id, signer_key_id),
+    FOREIGN KEY(master_key_id) REFERENCES
+    keyrings_public(master_key_id) ON DELETE CASCADE
+);