[prism] Add internal/urns package

[lostluck]

Adding Initial internal/urns package.

Urns simply simplifies having all the various URNs available, as directly extracted from the protos. In some future time, this should migrate the rest of the SDK, but for now, it's internal to the runner.

Uber PR for all changes: #25391

See #24789

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[lostluck]

R: @jrmccluskey