Adding a box spatial coverage when registering a resource from an external (unsupported) repository causes it to not be registered with discovery

[horsburgh]

Description of the bug
When a box spatial coverage is added to a resource from an external repository when it is registered with the DSP, it registers fine and shows up on the users "My Submissions" page, but it does not get registered with discovery because of a JSON validation error. @pkdash verified this via logs.

Steps to reproduce the bug:

1. Register a new resource from an external repository
2. Add a box coverage with a name
3. Submit
4. Resources shows up on My Submissions, but is not registered with discovery

Expected behavior
The JSON parsing error needs to be fixed so that valid metadata with box coverage can be registered.

Additional information
NA

[pkdash]

This bug seems to be in this code block where it always defaults to point type coverage as the incoming json not likely to have a key 'type'. The code needs to check the coverage extent field names and decide the type of coverage:

https://github.com/cznethub/dspback/blob/develop/dspback/pydantic_schemas.py#L312-L326