[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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[istio-testing]

Hi @DerekTBrown. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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