Add support for configuring default namespace

[AlanGreene]

Changes

Resolves https://github.com/tektoncd/dashboard/issues/3297

When the Dashboard deployment's --default-namespace arg is set, this configures teh default namespace selected when loading the application instead of 'All Namespaces' being selected.

This behaviour will only be triggered when the Dashboard is loaded with no path on the URL, i.e. only for the root (which automatically redirects to the About page).

The reason for this limitation is that handling the default namespace configuration as part of the normal namespace processing would break the ability to select 'All Namespaces' from the namespace filter dropdown, as well as causing issues with URL consistency. It would also be impossible to differentiate between a user attempting to visit a namespaced page (e.g. PipelineRuns) with 'All Namespaces' selected, vs. with no selection which would be expected to use the default namespace instead.

/kind feature

Submitter Checklist

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

• [ ] Docs included if any changes are user facing
• [ ] Tests included if any functionality added or changed
• [ ] 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 (new features, significant UI changes, API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• [ ] Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

Add the ability to configure the default namespace selected in the UI instead of 'All Namespaces' when the user visits the application root (i.e. `/`).

[tekton-robot]

Skipping CI for Draft Pull Request. If you want CI signal for your change, please convert it to an actual PR. You can still manually trigger a test run with /test all

[AlanGreene]

/kind documentation /test all

[AlanGreene]

Some additional testing required to ensure compatibility between this new config option and the existing tenant namespaces behaviour, and no other unexpected side effects.

[AlanGreene]

Thanks Brian, updated those strings 👍

[tekton-robot]

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