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/override.go

@@ -0,0 +1,133 @@
+package helpers
+
+import (
+	"net"
+	"time"
+
+	"github.com/qdm12/golibs/logging"
+	"inet.af/netaddr"
+)
+
+func OverrideWithBool(existing, other *bool) (result *bool) {
+	if other == nil {
+		return existing
+	}
+	result = new(bool)
+	*result = *other
+	return result
+}
+
+func OverrideWithString(existing, other string) (result string) {
+	if other == "" {
+		return existing
+	}
+	return other
+}
+
+func OverrideWithStringPtr(existing, other *string) (result *string) {
+	if other == nil {
+		return existing
+	}
+	result = new(string)
+	*result = *other
+	return result
+}
+
+func OverrideWithInt(existing, other *int) (result *int) {
+	if other == nil {
+		return existing
+	}
+	result = new(int)
+	*result = *other
+	return result
+}
+
+func OverrideWithUint8(existing, other *uint8) (result *uint8) {
+	if other == nil {
+		return existing
+	}
+	result = new(uint8)
+	*result = *other
+	return result
+}
+
+func OverrideWithUint16(existing, other *uint16) (result *uint16) {
+	if other == nil {
+		return existing
+	}
+	result = new(uint16)
+	*result = *other
+	return result
+}
+
+func OverrideWithIP(existing, other net.IP) (result net.IP) {
+	if other == nil {
+		return existing
+	}
+	result = make(net.IP, len(other))
+	copy(result, other)
+	return result
+}
+
+func OverrideWithDuration(existing, other *time.Duration) (result *time.Duration) {
+	if other == nil {
+		return existing
+	}
+	result = new(time.Duration)
+	*result = *other
+	return result
+}
+
+func OverrideWithLogLevel(existing, other *logging.Level) (result *logging.Level) {
+	if other == nil {
+		return existing
+	}
+	result = new(logging.Level)
+	*result = *other
+	return result
+}
+
+func OverrideWithStringSlice(existing, other []string) (result []string) {
+	if other == nil {
+		return existing
+	}
+	result = make([]string, len(other))
+	copy(result, other)
+	return result
+}
+
+func OverrideWithUint16Slice(existing, other []uint16) (result []uint16) {
+	if other == nil {
+		return existing
+	}
+	result = make([]uint16, len(other))
+	copy(result, other)
+	return result
+}
+
+func OverrideWithIPNetsSlice(existing, other []net.IPNet) (result []net.IPNet) {
+	if other == nil {
+		return existing
+	}
+	result = make([]net.IPNet, len(other))
+	copy(result, other)
+	return result
+}
+
+func OverrideWithNetaddrIPsSlice(existing, other []netaddr.IP) (result []netaddr.IP) {
+	if other == nil {
+		return existing
+	}
+	result = make([]netaddr.IP, len(other))
+	copy(result, other)
+	return result
+}
+
+func OverrideWithIPPrefixesSlice(existing, other []netaddr.IPPrefix) (result []netaddr.IPPrefix) {
+	if other == nil {
+		return existing
+	}
+	result = make([]netaddr.IPPrefix, len(other))
+	copy(result, other)
+	return result
+}