…ed through the `${PROVIDER}_APPLICATION_CREDENTIALS` environment variable.

* Add support to read JSON credential files present in a directory which
  is passed through the `${PROVIDER}_APPLICATION_CREDENTIALS` environment
  variable for all (applicable) providers.

* Remove examples which suggest that cloud provider credentials can be
  passed through a JSON file since this will be deprecated in future releases.

* Improve error handling in various `SnapStore` related files for each
  cloud provider.

* Updated docs to discourage usage of JSON files to pass credentials.

* Housekeeping in `pkg/server/backuprestoreserver.go`.
  Changed an unused named function parameter to `_`, and removed a
  tautological if condition.

* Remove JSON method from the various ways to pass credentials section.

* Remove a confusing line about passing credentials as individual files.

* Fix comments documenting functions

* Enhance `docs/deployment/getting_started.md`

* Unnecessary argument for error string creation in `pkg/snapstore/utils.go` removed