Add the "speculationrules" destination

[vickiez]

This destination is used when fetching speculation rule sets, whether via the
Speculation-Rules HTTP response header or an external
<script type=speculationrules src> element. It is governed by the
speculation-rules-src CSP directive.

---

Supersedes #1841, whose commit is included here unchanged and with its
authorship preserved. On top of it, this PR:

• fixes the illustrative destination table's "" initiator rowspan, which was
  not grown when the new row was added, leaving the table a cell short
  (editorial);
• moves "speculationrules" out of the "no CSP directive" grouping and gives it
  the speculation-rules-src directive; and
• notes that the destination also covers external
  <script type=speculationrules src> elements, not just the header.

Note that HTML's "process the Speculation-Rules header" already creates
requests whose destination is "speculationrules", so this value is referenced
today without being defined here.

Related pull requests

• Add speculation rules prefetch html#11426 (merged) - defines the Speculation-Rules header, and
  creates requests with this destination.
• Integrate with speculation rules  w3c/webappsec-csp#808 - defines speculation-rules-src and maps this
  destination to it.
• Add external <script type=speculationrules> html#11697 - makes an external <script type=speculationrules src>
  fetch its rule set using this destination.

• At least two implementers are interested (and none opposed):
  • See Add speculation rules prefetch html#11426
• Tests are written and can be reviewed and commented upon at:
  • The header is covered by https://github.com/web-platform-tests/wpt/blob/c669bcf970fe50b56e0ee53de9c2097a012dafc7/speculation-rules/prefetch/resources/ruleset.py#L40-L43
  • CSP integration is covered alongside Integrate with speculation rules  w3c/webappsec-csp#808
• Implementation bugs are filed:
  • See Add speculation rules prefetch html#11426
• MDN issue is filed: "speculationrules" request destination missing mdn/content#40289
• The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

---

Preview | Diff