credentials requriements surface in many places thorughout corso:
they can be sourced from many locations (envs, files, manually),
and used in many more (cli, repo, kopia). This usage could blossom
into all kinds of duplicate structs sharing similar info. The goal
of this change is to centralize where credentials are declared
and managed, and how they then cascade out to other packages.
* validate required storage props (#85)
Centralizes validation of required storage config properties within
the storage package. Requiremens are checked eagerly at
configuration creation, and lazily at config retrieval.
Additionally, updates /pkg/storage tests to use suites
and assert funcs.
* add validation failure tests to storage
* add common config and encryption passwd
Adds a provider-independent configuration handler, and the
the encryption password config property. The password is used
to encrypt and decrypt the kopia repository properties file.
* fix corso_password in ci.yml
* actually use the corso password in testing
* replace passwd in ci.yml with a secret
* ci.yml secret typo fix
