Write tiles to S3 cache bucket

[KlaasH]

This is the code side of adding tile caching. #634 is the infrastructure side. See also the issue comment and branch on the main Tilegarden project.

The task is to add code to api.js so that, if there is a variable configured in the environment with the cache bucket name, it writes all tiles to that bucket, using the request path as the key

This depends on #634 insofar as it needs to use the cache bucket configured there, though it could be done in parallel by manually creating a bucket and supplying a variable with its name.

Note that I'm not sure it's possible to actually set up a cached endpoint in development (maybe related to #593), but it should at least be possible to have a local Tilegarden instance write tiles to S3.