rust: pci: add irq accessors

rust: pci: add irq accessors

These accessors can be used to retrieve a irq::Registration or a
irq::ThreadedRegistration from a pci device. Alternatively, drivers can
retrieve an IrqRequest from a bound PCI device for later use.

These accessors ensure that only valid IRQ lines can ever be registered.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Tested-by: Joel Fernandes <joelagnelf@nvidia.com>
Tested-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Daniel Almeida <daniel.almeida@collabora.com>
Link: https://lore.kernel.org/r/20250811-topics-tyr-request_irq2-v9-6-0485dcd9bcbf@collabora.com
Signed-off-by: Danilo Krummrich <dakr@kernel.org>