Add additional authentication options for each provider in `cmd/internal`

[inkel]

Currently each provider is opinionated in how it authenticates, for example:

• GCP needs you to authenticate via the gcloud CLI.
• AWS needs a valid profile in ~/.aws/config and ~/.aws/credentials.
• Azure needs authentication via the az CLI.

All these providers have other means of authentication; we should try and add those as well. This tangentially relates to #37.

THIS ISSUE NEEDS MORE RESEARCH AND DOCUMENTATION

### Tasks
- [ ] Add additional authentication options for GCP
- [ ] Add additional authentication options for AWS
- [ ] Add additional authentication options for Azure