Support on_demand policy for remote

[NickLarsenNZ]

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

I'd like to be able to make a pull-through cache, using the on_demand policy.
This plugin doesn't seem to support it.

Describe the solution you'd like
Add support for on_demand and streaming policies.

Describe alternatives you've considered
None

Additional context
Example POST and response:

curl -u user:password -X POST \
  http://localhost:8080/pulp/api/v3/remotes/maven/maven/ \
  -H "Content-Type: application/json" \
  -d '{
    "name": "maven-central",
    "url": "https://repo1.maven.org/maven2/",
    "policy": "on_demand"
  }'

{"policy":["\"on_demand\" is not a valid choice."]}

See also: pulp/pulp-cli-maven#170