Add Sentinel 1 subpackage

[scottyhq]

I'd love to get a STAC catalog generated for the Sentinel-1 RTC public dataset covering CONUS in AWS https://registry.opendata.aws/sentinel-1-rtc-indigo/

I put together the framework for this over in the stac-sentinel repo, but it seems like the current recommendation is to add a submodule here:
stac-utils/stac-sentinel#1 (comment)

I'm thinking of trying to follow the sentinel2 subpackage closely. For starters this would just cover the RTC dataset, but there are others out there that could likely be added later (e.g. GRD, SLCs https://registry.opendata.aws/sentinel-1/). Doesn't seem like anyone else is actively working on this right @matthewhanson @lossyrob ?

[lossyrob]

@scottyhq I don't believe anybody is actively working on this; we'll need to generate STAC for Sentinel 1 GRD products in the next few months, and would gladly contribute to the subproject if you got it rolling!

Following the sentinel 2 package is a good idea - I've made some changes to it over in #83, which I'm aiming to merge this week. That and landsat (after that PR) should give some solid starting points.

[gadomski]

Closed by #84, moved to https://github.com/stactools-packages/sentinel1.