opt: use URL for EXPLAIN (OPT,ENV)

[jseldess]

PR: cockroachdb/cockroach#38973

From release notes:

EXPLAIN (OPT,ENV) now returns a URL with the data encoded in the fragment portion. Opening the URL shows a page with the decoded data. Note that the data is processed in the local browser session and is never sent out. [#38973][#38973] {% comment %}doc{% endcomment %}