Support functions for header configuration with TUS plugin

[danilat]

In my team we are working with uppy and our own TUS protocol server implementation, currently we found a limitation with @uppy/tus and headers management.

We want to send the lastModified of each file in each request, we thought to send a custom header with that information by file, but we found that currently we can only set the same headers for all requests. We expected some support similar to the existing in xhr-upload https://uppy.io/docs/xhr-upload/#headers. It'd great to have something similar