Add AsnClaim and Asn

[jstudler]

Add ASN and AsnClaim support

Adds ASN (Autonomous System Number) management to the NetBox Operator, following the same claim/resource pattern already used for IP addresses, prefixes and IP ranges.

New CRDs

• Asn — represents a single ASN in NetBox.
• AsnClaim — claims an available ASN from a NetBox ASN Range via parentAsnRange (matched by range name) and creates an owned Asn CR. Status exposes the assigned asn and asnName.

Controllers

• AsnReconciler — reserves/updates the ASN in NetBox, finalizer-based cleanup (asn.netbox.dev/finalizer), lease-locks the parent ASN range while the ASN is not yet Ready, tracks managed custom fields via annotation, reports conditions and events.
• AsnClaimReconciler — allocates an ASN from the parent range, restores a previously assigned ASN by hash before falling back to a new allocation, syncs mutable fields down to the owned Asn, lease-locks per range to avoid races between concurrent claims.

NetBox client

New methods on NetboxCompositeClient: ReserveOrUpdateAsn, DeleteAsn, RestoreExistingAsnByHash, GetAvailableAsnByClaim, plus internal RIR resolution and ASN-range-by-name lookup. New ASN / ASNClaim models and regenerated mocks for the IpamAsns* / IpamAsnRanges* API surface.

Tests & docs

• Unit tests for controllers, helpers and the NetBox client.
• Chainsaw e2e tests under tests/e2e/asn: apply/update, restore, range-exhausted and owner-reference cases; local test data (RIR + ASN ranges) seeded in the kind data-load job.
• README section "ASN Management" documenting the Asn/AsnClaim model, parentAsnRange semantics and restoration.

Behaviour notes

• Restoration hash makes ASN assignment sticky across CR re-creation; a hash mismatch on a not-yet-reserved Asn deletes the CR.
• preserveInNetbox: true keeps the ASN in NetBox after CR deletion, enabling later reclaim.
• asn and tenant immutability is enforced by CEL validation rules.

Open Questions

• Use Name of Parent ASN Range (e.g. "E2E Test ASN Range") or the actual range (e.g. "64512-64612")?
• Inherit RIR from parent ASN Range or not?