sql: use a closure to wrap transactions (#469)

This introduces the a `WithTransaction` function, which is then used for all transactional work in Steve.

Because `WithTransaction` takes care of all `Begin`s, `Commit`s and `Rollback`s, it eliminates the problem where forgotten open transactions can block all other operations (with long stalling and `SQLITE_BUSY` errors).

This also:

- merges together the disparate `DBClient` interfaces in one only `db.Client` interface with one unexported non-test implementation. I found this much easier to follow
- refactors the transaction package in order to make it as minimal as possible, and as close to the wrapped `sql.Tx` and `sql.Stmt` functions as possible, in order to reduce cognitive load when working with this part of the codebase
- simplifies tests accordingly
- adds a couple of known files to `.gitignore`
    
Credits to @tomleb for suggesting the approach: https://github.com/rancher/lasso/pull/121#pullrequestreview-2515872507

pkg/sqlcache/informer/informer.go

@@ -8,6 +8,7 @@ import (
 	"context"
 	"time"
 
+	"github.com/rancher/steve/pkg/sqlcache/db"
 	"github.com/rancher/steve/pkg/sqlcache/partition"
 	sqlStore "github.com/rancher/steve/pkg/sqlcache/store"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
@@ -34,7 +35,7 @@ var newInformer = cache.NewSharedIndexInformer
 
 // NewInformer returns a new SQLite-backed Informer for the type specified by schema in unstructured.Unstructured form
 // using the specified client
-func NewInformer(client dynamic.ResourceInterface, fields [][]string, transform cache.TransformFunc, gvk schema.GroupVersionKind, db sqlStore.DBClient, shouldEncrypt bool, namespaced bool) (*Informer, error) {
+func NewInformer(client dynamic.ResourceInterface, fields [][]string, transform cache.TransformFunc, gvk schema.GroupVersionKind, db db.Client, shouldEncrypt bool, namespaced bool) (*Informer, error) {
 	listWatcher := &cache.ListWatch{
 		ListFunc: func(options metav1.ListOptions) (runtime.Object, error) {
 			a, err := client.List(context.Background(), options)