Derive context to group secrets by project

The realm system allows to group secrets for organizing and avoiding name clashes.

Terminology: realm == namespace

Examples:

Let's say you have two projects (git repos), and in each there needs to be a secret named foo. Then these options exists:

my-project/foo
my-project/bar
vs
my-other-project/foo
vs
foo
vs
root/foo

Also the pattern matching logic build in the secrets engine allows to then filter all secrets per project, e.g., my-project/* only gives all secrets of project my-project.

Thus, we suggest the following CLI ux:

• there's a --global flag available for any docker mysecret subcommand
• if --global is set, no realm is appended/prefixed
• if --global is not set (default), the outer git repo is used to namespace / realm-prefix all set/list/delete/get operations
• if --global is not set and there's no outer git repo, the CLI errors