Support an optional per-call pubkey_fingerprint on the base SigningService.sign()

Is your feature request related to a problem? Please describe.

Today, metadata signing services bind one key per service. The base SigningService.sign() signs with whatever key the service was created with, with no per-call override. Both metadata services inherit this unchanged: AsciiArmoredDetachedSigningService (pulpcore, RPM repomd.xml) and AptReleaseSigningService (pulp_deb, deb Release). Package signing already solved this — RpmPackageSigningService.sign() takes a pubkey_fingerprint and injects it as PULP_SIGNING_KEY_FINGERPRINT, so one service signs for many keys.

As a result, signing repositories' metadata with different keys requires a separate signing service per key. In containerized deployments each new key means a new add-signing-service registration (on-disk script, importable GPG key, full validate() run) — effectively a deployment change per key. This is the pain behind pulpcore #6733 and pulp/pulp_rpm#4052.

Describe the solution you'd like

Mirror the package-signing pattern on the shared base method so both metadata services benefit:

1. Add an optional pubkey_fingerprint=None kwarg to the base SigningService.sign() (and asign()). When set, it overrides PULP_SIGNING_KEY_FINGERPRINT; when unset, it falls back to the service's own key (today's behavior). AsciiArmoredDetachedSigningService and AptReleaseSigningService inherit this automatically, since neither overrides sign().
2. Plugins add a nullable metadata_signing_fingerprint on their repository models (parallel to package_signing_fingerprint) and forward it from the publish task.

A deployment then registers one generic metadata signing service per format at boot and selects the key per publish via the repository's fingerprint.

FWIW - this solution is backwards compatible: _env_variables already injects PULP_SIGNING_KEY_FINGERPRINT, so this only overrides a value already passed; the kwarg is optional and defaults to None (unlike RpmPackageSigningService.sign(), which made it mandatory); the repo field is nullable; and validate() is unchanged since it signs with no fingerprint.

Describe alternatives you've considered

• A writable signing-service create/delete REST API (pulpcore #6733). Still one service per key, still carries the on-disk-script + keyring + validate() side effects, and re-opens the arbitrary-script-path security concern that stalled that issue.
• Invoking add-signing-service at runtime. Operationally fragile, couples callers to Pulp's pod internals, and produces a sprawl of per-key services.
• Keeping the fingerprint purely in the plugin (as pulp/pulp_rpm#4052 leaned). The repo field is inert unless the base sign() honors a per-call fingerprint — which is why this pulpcore change is the prerequisite.

Start at the base SigningService.sign() and asign() methods, then compare RpmPackageSigningService.sign() and its PULP_SIGNING_KEY_FINGERPRINT handling. Done means an optional fingerprint overrides the service environment value while None preserves current behavior, with inherited metadata signing services benefiting without their own sign() overrides.