use login and password instead of "api access key" in REST API
this request is extracted from #8529
it would be great if REST API supported the concept of "login" so that REST users would be able to "login" through REST API using their login&password (instead of API access key!) and then they'd receive some "session key" they could use temporarily for this session.
See how the same mechanism is implemented in Atlassian Jira remote API.
This would simplify the REST API usage in various UIs (people are often confused about "api access key", they want to use their login&password). Just imagine how inconvenient it is to type an API access key in Redmine Android client on a phone screen!