agent: allow changing file ownership in file sink

[clwluvw]

Allow changing the ownership of the token file in file sink.

[VioletHynes]

Hi there @clwluvw ! Just one little thing before we get this merged:

the linter check failed with the following: command/agentproxyshared/sink/file/file_sink_test.go:139:1: Test TestFileSinkMode_Ownership is missing a go doc

Could you make sure to add a godoc for the test, describing what it does and what it's for? Thanks :D

I also need to run this by @schavis to make sure there are no outstanding requested changes.

[clwluvw]

Hi @VioletHynes - Thanks for the review. I just added the godoc for the test.

[VioletHynes]

Thanks! It's a holiday in some parts of the world today and I'll need to confirm with Sarah before I merge this, but I hope to get this merged soon this week :)