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/downloader/client.go

@@ -0,0 +1,34 @@
+package downloader
+
+import (
+	"context"
+
+	"go.mau.fi/mautrix-telegram/pkg/gotd/tg"
+)
+
+// CDN represents Telegram RPC client to CDN server.
+type CDN interface {
+	UploadGetCDNFile(ctx context.Context, request *tg.UploadGetCDNFileRequest) (tg.UploadCDNFileClass, error)
+}
+
+// Client represents Telegram RPC client.
+type Client interface {
+	UploadGetFile(ctx context.Context, request *tg.UploadGetFileRequest) (tg.UploadFileClass, error)
+	UploadGetFileHashes(ctx context.Context, request *tg.UploadGetFileHashesRequest) ([]tg.FileHash, error)
+
+	UploadReuploadCDNFile(ctx context.Context, request *tg.UploadReuploadCDNFileRequest) ([]tg.FileHash, error)
+	UploadGetCDNFileHashes(ctx context.Context, request *tg.UploadGetCDNFileHashesRequest) ([]tg.FileHash, error)
+
+	UploadGetWebFile(ctx context.Context, request *tg.UploadGetWebFileRequest) (*tg.UploadWebFile, error)
+}
+
+type chunk struct {
+	data []byte
+	tag  tg.StorageFileTypeClass
+}
+
+// schema is simple interface for different download schemas.
+type schema interface {
+	Chunk(ctx context.Context, offset int64, limit int) (chunk, error)
+	Hashes(ctx context.Context, offset int64) ([]tg.FileHash, error)
+}