Store schema of custom entity and validate KongCustomEntity against Kong gateway

[randmonkey]

Is there an existing issue for this?

• I have searched the existing issues

Does this enhancement require public documentation?

• I have added an Acceptance Criteria item for adding and/or adjusting public documentation (if applicable)

Problem Statement

Split from #4556.
To validate a KongCustomEntity CR, we need to know the schema of "supported" entity in Kong gateway to

• Validate its type
• Fill in "foreign" fields to send it to /schemas/{type}/validate endpoints of validating

Proposed Solution

• Maintain a map to store schema of "Known" Kong entities
• When a KongCustomEntity CR created/updated, check its schema and store it
• In validating webhooks, fill in the "foreign" fields according to the stored schema.

Additional information

No response

Acceptance Criteria

• KIC has the storage for schema of supported Kong entities
• For each KongCustomEntity CR, its entity type is stored in KIC if the type exists in Kong gateway
• KIC can fill in the "foreign" fields of entity according to stored schema