[fixes #56772] Fix WasmPlugin .spec.url validation

[DerekTBrown]

What does this PR do?

• Per the Istio documentation, if the oci:// prefix is missing in .spec.url, it is assumed that the URL is a oci:// image reference¹.
• Things like foo.bar:5000/baz:latest are valid image references, but do not pass validation, because this treats foo.bar as the schema.
• I added an extensive set of test cases, then modified the validation logic to support each.

Footnotes

1. https://istio.io/latest/docs/reference/config/proxy_extensions/wasm-plugin/#WasmPlugin-url. ↩

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Hi @DerekTBrown. Thanks for your PR.

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