Allow custom vtgate/vtctld Service names

Currently, the vtgate Service name has a unique, deterministic hash suffix. This was part of a strategy to avoid collisions in the multi-tenant environment for which our operator was initially designed. For those running the operator in their own, controlled environments, the hash suffix makes it unnecessarily difficult to configure apps to connect to Vitess.

We should make the hash suffix optional for at least the main vtgate/vtctld Services, and possibly also the per-cell vtgate Services. Ideally we would also make the suffix disabled by default, but unfortunately that would be a backwards-incompatible change for those of us already running the operator in production.

To avoid over-constraining the user's choice, we could support this by adding an optional field to override the default Service name to a custom value.

Start by locating the operator entry points that construct the main vtgate and vtctld Service names, then inspect how per-cell vtgate Services and user-facing configuration are represented. Clarify whether the change should use an optional custom name, an optional hash suffix, or both, while preserving current defaults; done means the selected Services can be addressed by the configured names without breaking existing deployments.