Alerting: Create abstraction for launching transactions and refactor existing transaction management to use it (#46216)

* Remove InTransaction from RuleStore and make it its own interface

* Ensure that ctx-based is clear from name

* Resolve merge conflicts

* Refactor tests to work in terms of the introduced abstraction rather than concrete dbstore

pkg/services/ngalert/store/transactions.go

@@ -0,0 +1,12 @@
+package store
+
+import "context"
+
+// TransactionManager represents the ability to issue and close transactions through contexts.
+type TransactionManager interface {
+	InTransaction(ctx context.Context, work func(ctx context.Context) error) error
+}
+
+func (st *DBstore) InTransaction(ctx context.Context, f func(ctx context.Context) error) error {
+	return st.SQLStore.InTransaction(ctx, f)
+}