Add support for requesting a lease by device name directly

[bkhizgiy]

Replace the single selector field with a oneof exporter_selection, allowing leases to target either exporters via label selector or a specific device by resource name.
Both options are immutable to preserve lease semantics.

The field is no longer marked as REQUIRED, as the controller now validates whether a value was provided.

Issue: https://github.com/jumpstarter-dev/jumpstarter/issues/638
Related to: jumpstarter-dev/jumpstarter-controller#208

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