feat: introduced support for the s3 proxying

[alekc]

This is probably should not be merged for now, I am bringing it here to show how we fixed things following our discussion on slack.

To reassume things:

• current version of sdk doesn't handle webidentity when used on kubernetes through tokens requiring update
• once updated due to aws/aws-sdk-go@3f5879a goiardi will no longer be able to create a proper signed urls for cookbook uploads because knife sends malformed host parameter when downloading stuff.

I left current behaviour as it is, and introduced a new config var use-s3-proxy, which permits to upload cookbooks directly through /filestore/ endpoint.

Initial tests went well, but if this is merged it's going to be a breaking change for users.