move gotd fork into repo. (#111)

- update to latest telegram layer
- remove some references to fields in tg.Entities that don't exist in
the schema
- originally added here:
https://github.com/beeper/td/commit/820929062a2ba0104397bc01235ab58a9cff780e
  - referenced here
-
https://github.com/mautrix/telegramgo/commit/124f0967ed195b5a380c9bd02e170ada9710dde3
-
https://github.com/mautrix/telegramgo/commit/4205047aab2e0639217148b5d125bfaab668bd8e

pkg/gotd/telegram/peers/broadcast.go

@@ -0,0 +1,71 @@
+package peers
+
+import (
+	"context"
+
+	"github.com/go-faster/errors"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/tg"
+)
+
+// Broadcast is a broadcast Channel.
+type Broadcast struct {
+	Channel
+}
+
+// Signatures whether signatures are enabled (channels).
+func (b Broadcast) Signatures() bool {
+	return b.raw.GetSignatures()
+}
+
+// DiscussionGroup returns linked chat, if any.
+func (b Broadcast) DiscussionGroup(ctx context.Context) (Channel, bool, error) {
+	full, err := b.FullRaw(ctx)
+	if err != nil {
+		return Channel{}, false, err
+	}
+
+	id, ok := full.GetLinkedChatID()
+	if !ok {
+		return Channel{}, false, nil
+	}
+
+	ch, err := b.m.GetChat(ctx, id)
+	if err != nil {
+		return Channel{}, false, err
+	}
+
+	actual, ok, err := ch.ActualChat(ctx)
+	if err != nil {
+		return Channel{}, false, err
+	}
+	if !ok {
+		return Channel{}, false, errors.Errorf("chat %d is linked, but not migrated", id)
+	}
+
+	return actual, true, nil
+}
+
+// SetDiscussionGroup associates a group to a channel as discussion group for that channel.
+func (b Broadcast) SetDiscussionGroup(ctx context.Context, p tg.InputChannelClass) error {
+	if _, err := b.m.api.ChannelsSetDiscussionGroup(ctx, &tg.ChannelsSetDiscussionGroupRequest{
+		Broadcast: b.InputChannel(),
+		Group:     p,
+	}); err != nil {
+		return errors.Wrap(err, "set discussion group")
+	}
+
+	return nil
+}
+
+// ToggleSignatures enable/disable message signatures in channels.
+func (b Broadcast) ToggleSignatures(ctx context.Context, enabled bool) error {
+	if _, err := b.m.api.ChannelsToggleSignatures(ctx, &tg.ChannelsToggleSignaturesRequest{
+		Channel:           b.InputChannel(),
+		SignaturesEnabled: enabled,
+	}); err != nil {
+		return errors.Wrap(err, "toggle signatures")
+	}
+
+	return nil
+}