✨ [amp-consent] Accept purposeConsentMap from external consent UI flow

[micajuine-ho]

Partial for #31607

Allow our external consent flow (from promptUISrc) accept a purposeConsents within the consent decision post message:

          parent.postMessage(
             {
                type, 
                action,
                info,
                consentMetadata,
                purposeConsents, /* new */
              },
              "*"
           );

If present, the map will be validated and stored in the consent-state-manager, iff the consent action was ACCEPT or REJECT.
We are only receiving the purposeConsents right now, we will send them to the 3p consent dialog iframe once we store.

Ignore whitespace changes to examples/amp-consent/diy-consent.html.