[feature] support nested configuration files, and setting ALL configuration variables by CLI and env (#4109)

This updates our configuration code generator to now also include map marshal and unmarshalers. So we now have much more control over how things get read from pflags, and stored / read from viper configuration. This allows us to set ALL configuration variables by CLI and environment now, AND support nested configuration files. e.g.

```yaml
advanced:
    scraper-deterrence = true

http-client:
    allow-ips = ["127.0.0.1"]
```

is the same as

```yaml
advanced-scraper-deterrence = true

http-client-allow-ips = ["127.0.0.1"]
```

This also starts cleaning up of our jumbled Configuration{} type by moving the advanced configuration options into their own nested structs, also as a way to show what it's capable of. It's worth noting however that nesting only works if the Go types are nested too (as this is how we hint to our code generator to generate the necessary flattening code :p).

closes #3195

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4109
Co-authored-by: kim <grufwub@gmail.com>
Co-committed-by: kim <grufwub@gmail.com>

internal/config/util.go

@@ -0,0 +1,74 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package config
+
+import (
+	"fmt"
+
+	"codeberg.org/gruf/go-split"
+	"github.com/spf13/cast"
+)
+
+func toStringSlice(a any) ([]string, error) {
+	switch a := a.(type) {
+	case []string:
+		return a, nil
+	case string:
+		return split.SplitStrings[string](a)
+	case []any:
+		ss := make([]string, len(a))
+		for i, a := range a {
+			var err error
+			ss[i], err = cast.ToStringE(a)
+			if err != nil {
+				return nil, err
+			}
+		}
+		return ss, nil
+	default:
+		return nil, fmt.Errorf("cannot cast %T to []string", a)
+	}
+}
+
+func mapGet(m map[string]any, keys ...string) (any, bool) {
+	for len(keys) > 0 {
+		key := keys[0]
+		keys = keys[1:]
+
+		// Check for key.
+		v, ok := m[key]
+		if !ok {
+			return nil, false
+		}
+
+		if len(keys) == 0 {
+			// Has to be value.
+			return v, true
+		}
+
+		// Else, it needs to have
+		// nesting to keep searching.
+		switch t := v.(type) {
+		case map[string]any:
+			m = t
+		default:
+			return nil, false
+		}
+	}
+	return nil, false
+}