ULNRemotes expose username/password in the serializer.

[ggainey]

Version
rpm/3.18-dev
core/3.19-dev

Describe the bug
Serializers shouldn't expose sensitive information. UlnRemoteSerializer, alas, does.

To Reproduce

http POST :/pulp/api/v3/remotes/rpm/uln/ name=ulnremote url=uln://uln username=foo password=bar
{
    "ca_cert": null,
    "client_cert": null,
    "connect_timeout": null,
    "download_concurrency": null,
    "headers": null,
    "max_retries": null,
    "name": "ulnremote",  
    "password": "bar",      <========== PASSWORD EXPOSED
    "policy": "immediate",
    "proxy_url": null,
    "pulp_created": "2022-03-18T12:21:51.029375Z",
    "pulp_href": "/pulp/api/v3/remotes/rpm/uln/3ddb3442-df7a-4845-81dc-4286b716daa5/",
    "pulp_labels": {},
    "pulp_last_updated": "2022-03-18T12:21:51.029391Z",
    "rate_limit": null,
    "sock_connect_timeout": null,
    "sock_read_timeout": null,
    "tls_validation": true,
    "total_timeout": null,
    "uln_server_base_url": null,
    "url": "uln://uln",
    "username": "foo"   <========== USERNAME EXPOSED
}

Expected behavior
We can't expose sensitive information in the serializers. UlnRemote needs to protect username/password like the default remote.

Additional context
UlnRemote needs some work, it overrides fields from base Remote that it doesn't need to. If UlnRemote didn't have its "own" username/password, this bug wouldn't exist.