* add --all and --data to backup create
Adds flags for backing up all exchange data, and
for isolating the data in the backup by data type.
Introduces validation and selector creation in
backup create. Switches the --user flag variable
type from a string to a []string.
* Add the backup list command to the cli
Adds `corso backup list <service>` to the cli commands.
Currently does nothing, while we're waiting on
downstream implementation. e2e hookup will arrive later.
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.
