Provide access to the admission.Request in validating & defaulting webhooks

[sbueringer]

As a developer writing defaulting and validating webhooks I would like to have access to the admission.Request without having to write an entire admission.Handler.

Our specific use case in Cluster API is that we want to validate an object on update depending on if the admission request is a dryrun.

Other use cases:

• Leverage request.UserInfo to block requests

Possible solution:

• Expose admission.Request in the context.Context passed into the Default/Validate funcs in the CustomDefaulter/CustomValidator (and a corresponding util func to read it from there)
  • Idea is to avoid changing the Validator/CustomValidator Defaulter/CustomValidator again / introducing new ones

[vincepri]

+1 to pass and adding utilities for the context in CustomDefaulter/Validator to access this information

[sbueringer]

Potential implementation in #1950