feat(ws): Notebooks 2.0 // Backend // List namespaces #53

[ederign]

Namespaces listing will be used to create a workspace and on the test app to create a namespace selector.

In this PR:

• handler and repository to fetch namespaces.
• rename the workspacekinds_handler test to align with the main file name.

• The screenshot lists all namespaces, but we should restrict to namespaces that the user can access in a fup PR.

[google-oss-prow]

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