Add configurable timeout for pbm backup

Description

While creating new backups, I have randomly faced timeout issues. Specifically: get backup metadata: get: context deadline exceeded which can be tracked to https://github.com/percona/percona-backup-mongodb/blob/4aca65bc1e2655966337790270ea2e0edbcada2b/cmd/pbm/backup.go#L158.

While the operation technically fails, the backup is created correctly. So I’m thinking: maybe the cli just needs to wait for a little longer sometimes?

In order to do so, I would like to be able to configure the timeout value used in startCtx https://github.com/percona/percona-backup-mongodb/blob/4aca65bc1e2655966337790270ea2e0edbcada2b/cmd/pbm/backup.go#L156 which gets propagated all the way serving as the timeout for the whole operation and is currently a fixed value: https://github.com/percona/percona-backup-mongodb/blob/4aca65bc1e2655966337790270ea2e0edbcada2b/pbm/defs/defs.go#L54

Acceptance criteria

There’s a way to configure the timeout for the pbm backup command.