Mark Laing discovered in LXD's PKI mode, until version 5.21.1, that a restricted certificate could be added to the trust store with its restrictions not honoured.

Affected: Go: github.com/canonical/lxd

### Summary If a `server.ca` file is present in `LXD_DIR` at LXD start up, LXD is in "PKI mode". In this mode, all clients must have certificates that have been signed by the CA.

The LXD configuration option `core.trust_ca_certificates` defaults to `false`. This means that although the client certificate has been signed by the CA, LXD will additionally add the certificate to the trust store and verify it via mTLS.

When a restricted certificate is added to the trust store in this mode, it's restrictions are not honoured, and the client has full access to LXD.

### Details When authorization was refactored to allow for generalisation (at the time for TLS, RBAC, and OpenFGA, see https://github.com/canonical/lxd/pull/12313), PKI mode did not account for the `core.trust_ca_certificates` configuration option. When this option is enabled, all CA-signed client certificates are given full access to LXD. [This cherry-pick from Incus](https://github.com/canonical/lxd/pull/12513/commits/5cdc9a35b9c51e981b1e70330bde0413ccacc7fd) was added to LXD to fix the issue.

The cherry-pick fixed the immediate issue and allowed full access to LXD for CA-signed client certificates when `core.trust_ca_certificates` is enabled, but did not consider the behaviour of LXD when `core.trust_ca_certificates` is disabled.

When `core.trust_ca_certificates` is false, restrictions that are applied to a certificate should be honoured. Instead, they are being ignored due to the presence of a `server.ca` fil…

govulncheck-vulndb-0.0.20241209T183251-1.1 on GA media

Affected: Ubuntu:Pro:16.04:LTS: lxd, Ubuntu:Pro:18.04:LTS: lxd, Ubuntu:20.04:LTS: lxd

Mark Laing discovered in LXD's PKI mode, until version 5.21.1, that a restricted certificate could be added to the trust store with its restrictions not honoured.