Add support for environmental secrets

[breca]

This merge adds support for type=env secret parameter, picking up the work of @71ms1 in #671.

It also adds some basic tests to the existing secret checks:

[test] podman start -a secrets_test_1
[test] | -rw-rw-r--    1 root     root            30 Feb 24 08:15 /run/secrets/custom_name
[test] | -rw-rw-r--    1 root     root            30 Feb 24 08:15 /run/secrets/file_secret
[test] | -r--r--r--    1 root     root             4 Feb 24 10:10 /run/secrets/my_secret
[test] | -r--r--r--    1 root     root             4 Feb 24 10:10 /run/secrets/my_secret_2
[test] | -r--------    1 103      103              4 Feb 24 10:10 /run/secrets/my_secret_3
[test] | -rw-rw-r--    1 root     root            30 Feb 24 08:15 /run/secrets/unused_params_warning
[test] | -rw-rw-r--    1 root     root            30 Feb 24 08:15 /etc/custom_location
[test] | important-secret-is-important
[test] | important-secret-is-important
[test] | sec
[test] | sec
[test] | sec
[test] | important-secret-is-important
[test] | important-secret-is-important
[test] | ENV_SECRET=sec
[test] exit code: 0