Add validation for remote URL

Author: @ipanova (ipanova@redhat.com)

Redmine Issue: 8340, https://pulp.plan.io/issues/8340

It would be good to add any sort of validation for the url. Right now we are not checking even the validity of the url schema.

(pulp) [vagrant@pulp2-nightly-pulp3-source-centos7 ~]$ http POST :24817/pulp/api/v3/remotes/rpm/rpm/  name=lala url=lala
HTTP/1.1 201 Created
Access-Control-Expose-Headers: Correlation-ID
Allow: GET, POST, HEAD, OPTIONS
Connection: close
Content-Length: 529
Content-Type: application/json
Correlation-ID: 22466def5b17436386ce6286f58b0307
Date: Thu, 04 Mar 2021 12:07:36 GMT
Location: /pulp/api/v3/remotes/rpm/rpm/2d554142-faa0-4fe6-bea5-83082806ac03/
Server: gunicorn/20.0.4
Vary: Accept, Cookie
X-Frame-Options: SAMEORIGIN

{
    "ca_cert": null,
    "client_cert": null,
    "client_key": null,
    "connect_timeout": null,
    "download_concurrency": 10,
    "name": "lala",
    "password": null,
    "policy": "immediate",
    "proxy_url": null,
    "pulp_created": "2021-03-04T12:07:36.223237Z",
    "pulp_href": "/pulp/api/v3/remotes/rpm/rpm/2d554142-faa0-4fe6-bea5-83082806ac03/",
    "pulp_labels": {},
    "pulp_last_updated": "2021-03-04T12:07:36.223260Z",
    "rate_limit": null,
    "sles_auth_token": null,
    "sock_connect_timeout": null,
    "sock_read_timeout": null,
    "tls_validation": true,
    "total_timeout": null,
    "url": "lala",
    "username": null
}

Start with the remote creation API shown in the issue and trace how the remote URL is accepted. Reproduce the POST with url=lala, then add validation so an invalid URL schema is rejected and verify that valid remote URLs continue to work.