Add per-resolution url parameter to Hub Resolver

[ab-ghosh]

Changes

Implements Option A from #9353: adds a url parameter to the Hub Resolver that allows Pipeline authors to override the hub API endpoint on a per-resolution basis, instead of relying solely on the cluster-wide ARTIFACT_HUB_API or
TEKTON_HUB_API environment variables.

• Adds a new optional url param that accepts an absolute HTTP/HTTPS URL
• When specified, overrides ARTIFACT_HUB_API or TEKTON_HUB_API based on type
• When omitted, falls back to the cluster-configured default
• Validates the URL scheme

Option B (multiple default URLs with fallback at the cluster level) will be addressed in a follow-up PR

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
• pre-commit Passed
• Follows the commit message standard
• Meets the Tekton contributor standards (including functionality, content, code)
• Has a kind label. You can add one by adding a comment on this PR that contains /kind <type>. Valid types are bug, cleanup, design, documentation, feature, flake, misc, question, tep
• Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings). See some examples of good release notes.
• Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

 Hub Resolver now supports an optional 'url' parameter to override the hub API endpoint per resolution, enabling use of internal hub instances.

[tekton-robot]

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[ab-ghosh]

/kind feature

[ab-ghosh]

Closing this PR in favor of a single PR that implements Option C (Option A + Option B combined) as discussed in #9353.