Use AbslStringify in StringBuilder for custom types

Replace use of absl::StrCat with absl::AbslStringify within the
StringBuilder class. This introduces a StringBuilderSink and
AbslFormatFlush to allow types that support AbslStringify to be appended
directly to the underlying string buffer. By not using absl::StrCat for
these types, the builder avoids creating intermediate string objects.

Bug: none
Change-Id: I9da8cbe64aebed8f1cb949ed7f81b242b5a88385
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/468620
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Reviewed-by: Evan Shrubsole <eshr@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47599}