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/dcs/find.go

@@ -0,0 +1,60 @@
+package dcs
+
+import (
+	"sort"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/tg"
+)
+
+// FindDCs searches DCs candidates from given config.
+func FindDCs(opts []tg.DCOption, dcID int, preferIPv6 bool) []tg.DCOption {
+	// Preallocate slice.
+	candidates := make([]tg.DCOption, 0, 32)
+
+	for _, candidateDC := range opts {
+		if candidateDC.ID != dcID {
+			continue
+		}
+		candidates = append(candidates, candidateDC)
+	}
+
+	if len(candidates) < 1 {
+		return nil
+	}
+
+	sort.Slice(candidates, func(i, j int) bool {
+		l, r := candidates[i], candidates[j]
+
+		// If we prefer IPv6 and left is IPv6 and right is not, so then
+		// left is smaller (would be before right).
+		if preferIPv6 {
+			if l.Ipv6 && !r.Ipv6 {
+				return true
+			}
+			if !l.Ipv6 && r.Ipv6 {
+				return false
+			}
+		}
+
+		// Also we prefer static addresses.
+		return l.Static && !r.Static
+	})
+
+	return candidates
+}
+
+// FindPrimaryDCs searches new primary DC from given config.
+// Unlike FindDC, it filters CDNs and MediaOnly servers, returns error
+// if not found.
+func FindPrimaryDCs(opts []tg.DCOption, dcID int, preferIPv6 bool) []tg.DCOption {
+	candidates := FindDCs(opts, dcID, preferIPv6)
+	// Filter (in place) from SliceTricks.
+	n := 0
+	for _, opt := range candidates {
+		if !opt.MediaOnly && !opt.CDN {
+			candidates[n] = opt
+			n++
+		}
+	}
+	return candidates[:n]
+}