Signed URLs for tiles endpoint and ipyleaflet tile viewer

[banesullivan]

These changes enable support for embedding the authentication token as a query parameter. This is for use cases where we want to "pre-sign" a URL for handing off to 3rd party software. Specifically, this is for the use case of generating a pre-signed tile URL for an image so that we can visualize it interactively with ipyleafelt.

@mvandenburgh and @AlmightyYakob, would you please review the changes to the authentication and python client?

---

Here is an example with a new helper method in the rgd_imagery_client:

from ipyleaflet import Map, projections, ScaleControl, FullScreenControl
from rgd_client import create_rgd_client

client = create_rgd_client(username='email@kitware.com', api_url='http://localhost:8000/api')

t = client.imagery.get_leaflet_tile_source(18, band=1, palette='matplotlib.Viridis_20', vmin=50, vmax=200)

m = Map(
        center=(37.7249511580583, -122.27230466902257), 
        zoom=9, crs=projections.EPSG3857, 
       )

m.add_layer(t)
m

[jjnesbitt]

@banesullivan and I discussed this offline, and we decided to change this approach to utilize presigned URLs, as including the entire auth token in the URL presents big security vulnerabilities. We came across this package, which seems to implement exactly what we're looking for. If nothing else it's a good starting point.

[banesullivan]

To mitigate these concerns, I added a new SignatureView to create expiry signatures per user and a new SignedURLRestViewMixin that can be added to downstream views which can use this SignedURLAuthentication that can be manually added to downstream view's authentication_classes.

At present, I only enable this functionality for the tiles endpoint and I think it should remain an opt-in feature. That way if someone gets ahold of a signature with malicious intent, they cannot use it to access any POST endpoints.

@mcovalt, would you please review?

[mcovalt]

This looks good!

[jjnesbitt]

I think it'd be good to include an expires_at param here, indicating the unix timestamp this signature expires as. I'll open an issue so it can be addressed separately.

[jjnesbitt]

#606