log/slog: remove calls to Value.Resolve from core

log/slog: remove calls to Value.Resolve from core

Remove calls to Value.Resolve from Record.AddAttrs, Record.Add and Logger.With.
Handlers must resolve values themselves; document that in Handler.

Call Value.Resolve in the built-in handlers.

Updates #59292.

Change-Id: I00ba2131be0b16e3b1a22741249fd6f81c3efde1
Reviewed-on: https://go-review.googlesource.com/c/go/+/486375
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>