encoding/json/v2: support format tag via runtime Option

This is an alternative workaround to CL 773980.

Instead of using a GOEXPERIMENT flag,
which only applies at compile-time,
use a special json.Options that enables
support for the `format` tag option at runtime.

This provides greater flexibility for logic
that is functionally already linked into the runtime
implementation of the "json" package.

The `format` flag is already widely used (proving its utility),
but it is not going to be included in the initial release of
"encoding/json/v2" since we want to implement it
using typed struct tags prospectively coming in Go 1.27.

This runtime option provides a workaround until then.

This CL fully removes support for JSON object names
expressed using the single-quoted string syntax.
Again, this is better supported in the future
with typed struct tags. Since there are no known usages
of this pattern, it is okay to remove this without a workaround.

Updates #79071

Change-Id: Ifb29e35a5e57cfbacc24e8ca2737911d9e654d23
Reviewed-on: https://go-review.googlesource.com/c/go/+/788420
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Damien Neil <dneil@google.com>