Policy: KAS registry remote public key should permit a port number within the remote key URI

[jakedoublev]

This line of regex denies a port number in a registered KAS's remote public key URI: https://github.com/opentdf/platform/blob/main/service/policy/objects.proto#L302

This should be valid: https://local.com:8080/kas/v2/kas_public_key but instead gives:

public_key.remote: URI must be a valid URL (e.g., 'https://demo.com/') followed by additional segments. Each segment must start and end with an alphanumeric character, can contain hyphens, alphanumeric characters, and slashes. [uri_format]

Acceptance Criteria

1. consider if this.isUri() is a better CEL validator for this need
2. port numbers are allowed
3. unit tests added to proto validation for this scenario:

   platform/service/policy/kasregistry/key_access_server_registry_test.go

   Line 208 in ec46a3a

   bad := []struct {