k8s gateway extensions: allow customizing translator

[stevenctl]

Description

API changes

The only API is an internal code API to allow GME to take control of the Gateway translation.

Changed CreatePluginRegistry to GetPluginRegistry. AFAICT there is no need to recreate the registry each time but I can rever tthis if needed.

	// GetPluginRegistry exposes the plugins supported by this implementation.
	GetPluginRegistry() registry.PluginRegistry

Added: GetTranslator to allow conditionally switching the translation logic based on the type of gateway.

	// GetTranslator allows an extnsion to provide custom translation for
	// different gateway classes.
	GetTranslator(*apiv1.Gateway) translator.K8sGwTranslator

Context

See slack conversation here

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• [] I have added tests that prove my fix is effective or that my feature works

[github-actions]

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[EItanya]

Changes LGTM, just want confirmation about the registry creation semantics for portal

[jmhbh]

We'll need a PR on the enterprise side to update the implementations of the interface after this PR merges otherwise LGTM