[DoctrineBridge] [ORM] Use custom cache namespace option if it is specified

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

This allows an option `namespace` to be used instead of having Symfony generate the hash. If the option is not set, the original behaviour will occur.

While this code will execute on the current version, this change depends on https://github.com/doctrine/DoctrineBundle/pull/198 to fully work.