Add config validation

[alyssajoyner]

Merge after: grafana/grafana#122302

Adds inline field validation to the ClickHouse v2 config editor behind the clickHouseConfigValidation feature toggle. When enabled, the following fields are validated on save and display inline errors:

• Server address (ServerAndEncryptionSection)
• Port (ServerAndEncryptionSection)
• Username (DatabaseCredentialsSection)

When validation fails, Grafana's native datasource testing status banner surfaces the errors — no save reaches the backend until all required fields are filled.
Without this, an empty host/port would reach the ClickHouse backend and return a generic connection error. Inline errors give users immediate, field-level feedback before any network request is made.

Notes

• Test this feature by setting clickHouseConfigValidation= true and newClickhouseConfigPageDesign = true
• When clickHouseConfigValidation = false (default), behaviour is unchanged — validation is undefined, no validators register, save proceeds as before.
• Validation only applies to the v2 config editor (newClickhouseConfigPageDesign = true).
• Requires the companion grafana/grafana PR that wires DataSourceConfigValidationAPI into EditDataSource and passes the validation prop to plugin config editors.