Backport: Only allow text/plain parser for REST API `device/validate`
This endpoint passes the request data to the from_string
method
of the Jinja sandboxed environment. from_string
only accepts
the string.
Use parser_classes
option to pass the PlainTextParser to the
action decorator.
This also makes browsable API to only render the simple text entry rather than allowing anyone to see the available device types and users through the choice boxes.
Upstream MR: https://gitlab.com/lava/lava/-/merge_requests/2250