Support configuring Discovery address in the TidbCluster CR

[DanielZhangQD]

Feature Request

Is your feature request related to a problem? Please describe:

For across Kubernetes deployment, we require the PD cluster in all of the clusters to join the clusters through one Discovery Pod in one cluster:

• One cluster deploys discovery as we do now
• The TidbCluster in the other clusters will configure the Discovery headless service address, and no Discovery is created in these clusters and PD Pods access the configured Discovery address to get the startup parameters.

Describe the feature you'd like:

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy:

[L3T]

/assign