encoding: add TextAppender and BinaryAppender
For #62384
Change-Id: I54707a29653df72ad9cd5633f434b87e0f630b94
GitHub-Last-Rev:
4f78947ac563d78f862c5c8de1c2e1578a8d6e08
GitHub-Pull-Request: golang/go#68620
Reviewed-on: https://go-review.googlesource.com/c/go/+/601595
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>