chore(settings): refactor settings processing (#756)

- Better settings tree structure logged using `qdm12/gotree`
- Read settings from environment variables, then files, then secret files
- Settings methods to default them, merge them and override them
- `DNS_PLAINTEXT_ADDRESS` default changed to `127.0.0.1` to use DoT. Warning added if set to something else.
- `HTTPPROXY_LISTENING_ADDRESS` instead of `HTTPPROXY_PORT` (with retro-compatibility)

internal/configuration/settings/helpers/pointers.go

@@ -0,0 +1,11 @@
+package helpers
+
+import "time"
+
+// StringPtr returns a pointer to the string value
+// passed as argument.
+func StringPtr(s string) *string { return &s }
+
+// DurationPtr returns a pointer to the duration value
+// passed as argument.
+func DurationPtr(d time.Duration) *time.Duration { return &d }