feat(client): opt-in fasthttp transport (NewFastHTTPDoer)

Adds an alternative HTTPDoer backed by valyala/fasthttp for high-throughput
bots. Cuts per-call allocs from 102 to 56 in the cross-library bench
(within 8 of telego, which uses fasthttp by default), and per-call bytes
from 11.1 KiB to 6.6 KiB.

  bot := client.New(token,
      client.WithHTTPClient(client.NewFastHTTPDoer()),
  )

Implementation notes:
  - Wraps *fasthttp.Client behind the existing HTTPDoer (Do *http.Request)
    interface, so RetryDoer, custom transports, observability middleware,
    and the 1428 generated tests all keep working as-is.
  - Translates *http.Request -> fasthttp.Request once per call and
    returns a *http.Response whose Body releases the pooled fasthttp
    response on Close (net/http contract).
  - Recognises the bufferReadCloser / readerReadCloser shapes produced
    by buildRequest and passes their underlying bytes straight to
    SetBodyRaw -- no io.ReadAll, no copy.
  - Honours ctx.Deadline via DoDeadline, falls back to WithFastHTTPReadTimeout
    when no deadline is set. fasthttp.ErrTimeout maps to
    context.DeadlineExceeded for errors.Is compatibility.

Default stays net/http: fasthttp is HTTP/1.1 only, doesn't compose with
the http.RoundTripper middleware ecosystem, and most users don't have
the throughput to notice. Bots making thousands of API calls/sec should
opt in.

Multipart/file-upload path remains on net/http per the agreed scope --
the perf bottleneck was JSON-method round-trip, not file uploads.

Time numbers in the report deferred until a quiet-system bench run;
allocs/bytes numbers (which are deterministic per code path) are
already updated.

test/benchmarks/call_bench_test.go

@@ -28,7 +28,8 @@ import (
 // Telegram-format token (digits:[\w-]{35}). telego enforces this format on construction.
 const benchToken = "1234567890:ABCDEFGHIJKLMNOPQRSTUVWXYZ_ab123456"
 
-// BenchmarkCall_ours — lukaszraczylo/go-telegram.
+// BenchmarkCall_ours — lukaszraczylo/go-telegram with default net/http
+// transport. Most users land here.
 func BenchmarkCall_ours(b *testing.B) {
 	srv := shared.NewMockServer()
 	defer srv.Close()
@@ -47,6 +48,30 @@ func BenchmarkCall_ours(b *testing.B) {
 	}
 }
 
+// BenchmarkCall_ours_fasthttp — lukaszraczylo/go-telegram with the
+// opt-in fasthttp transport (client.NewFastHTTPDoer). Apples-to-apples
+// against telego, which also runs on fasthttp by default.
+func BenchmarkCall_ours_fasthttp(b *testing.B) {
+	srv := shared.NewMockServer()
+	defer srv.Close()
+	bot := client.New(benchToken,
+		client.WithBaseURL(srv.URL),
+		client.WithHTTPClient(client.NewFastHTTPDoer()),
+	)
+	ctx := context.Background()
+	b.ReportAllocs()
+	b.ResetTimer()
+	for b.Loop() {
+		_, err := api.SendMessage(ctx, bot, &api.SendMessageParams{
+			ChatID: api.ChatIDFromInt(42),
+			Text:   "hello",
+		})
+		if err != nil {
+			b.Fatal(err)
+		}
+	}
+}
+
 // BenchmarkCall_gotba — go-telegram-bot-api/telegram-bot-api/v5.
 func BenchmarkCall_gotba(b *testing.B) {
 	srv := shared.NewMockServer()