Basic defaulting and validation for TidbCluster CR

[aylei]

Feature Request

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

#1121 is going to complete, we need server-side defaulting as a replacement of client-side defaulting as we used to do in helm. Also, validation, as a missing part, should also be added.

Describe the feature you'd like:

Possible solutions:

• AdmissionWebhook: validation and defaulting
• OpenAPIV3 validation schema: schema based validation and defaulting, the schema could be automatically generated for struct definition in Go, additional properties like validation and default value could be written in field comment. See here for details.
  • available in k8s 1.16 or higher (Validation was Beta from 1.15, Defaulting was Beta from 1.16, Both of them became GA in 1.17)
• Aggregated APIServer

Teachability, Documentation, Adoption, Migration Strategy:

This change is tend to be non-breaking. But the reference doc of API could be tedious to maintain, we have to figure out a solution to generate the API document from the CRD or Go code.