Add check API

[dmitriim]

As this plugin can be a critical path to the inf set up, would be useful to add some health check using check API.

Candidates:

• [ ] proxyrangerequests is enabled, but "Test range request failed"
• [ ] A number of missing files are growing
• [ ] a basic check which is that it can both connect to the store and grab a known small test file
• [ ] if cloudfront is on, check that a remote url work. also check the CORS headers are good

[brendanheywood]

+1 but lets not worry too much about getting them all done in this issue, we can split them up into different checks and issues

[dmitriim]

Yes. We can throw more ideas here and then tackle them separately.

[brendanheywood]

+1 to have a super basic check which is that it can both connect to the store, and do something simple like a head request for a file which is knows is in the store to make sure it works end to end