backend: Backend.StateMgr takes context.Context

This adds a new context.Context argument to the Backend.StateMgr method,
updates all of the implementations to match, and then updates all of the
callers to pass in a context.

A small number of callers don't yet have context plumbed to them so those
use context.TODO() as a placeholder for now, so we can more easily find
and fix them in later commits once we have contexts more thoroughly
plumbed.

Signed-off-by: Martin Atkins <mart@degeneration.co.uk>

internal/backend/remote-state/http/backend.go

@@ -303,7 +303,7 @@ func (b *Backend) configure(ctx context.Context) error {
 	return nil
 }
 
-func (b *Backend) StateMgr(name string) (statemgr.Full, error) {
+func (b *Backend) StateMgr(_ context.Context, name string) (statemgr.Full, error) {
 	if name != backend.DefaultStateName {
 		return nil, backend.ErrWorkspacesNotSupported
 	}