encoding: add AppendEncode and AppendDecode

encoding: add AppendEncode and AppendDecode

Implement append-like equivalent of Encode and Decode functions.

Fixes #53693

Change-Id: I79d8d834e3c8f77fad32be2fd391e33d4d1527ea
Reviewed-on: https://go-review.googlesource.com/c/go/+/504884
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>