Configurable file permissions

[johnabass]

Add configurable, nix-style file permissions for log files in both the standard and custom lumberjack sinks. This involves several items:

(1) Added a Permissions string field to sallust.Config. This field will have the same format as nix-style permissions, allowing a leading zero, e.g. "0644" and "644" are both acceptable. sallust will then parse that into an fs.FileMode.

(2) Add code that happens before the standard zap.Sink that creates the file with the configured permissions. This will probably have to happen in sallust.Config.Build(...).

(3) Include a perms=xyz parameter in the lumberjack URI for the sallust.Lumberjack type. This URI parameter's value has the same format(s) as in (1) above. This is required so that configuration information carries down to the custom code in sallust.NewLumberJackSink. See: https://pkg.go.dev/github.com/xmidt-org/sallust#NewLumberjackSink