✨ [amp-consent] Sync and store purposeConsents in localStorage

[micajuine-ho]

Partial for #31607

Alter what it means to have a stored value; we now must have consent decisions for every purpose consent listed in the purposeConsentRequired field (either inline or from remote response).

In the case of an already stored consent decision, sync new consent discussions (purposeConsents) via checkConsentHref, which will be reflected on next page visit.

Store the purposeConsents in local storage under the same namespace as the existing consent information under the key pc:

"vv": {
    "amp-consent:_ping_": {
      "v": {
        "s": 1,
        "r": "accept-string",
        "pc": {
          "purpose-ads": 1,
          "purpose-analyltics": 1,
          "purpose-analytics-vendor-a": 2
        }
      },
      "t": 1613518078945
    }
  }

Also did some cleaning up of tests (function* to preferred async/await pattern).

[dmanek]

You can remove this assignment. If nothing is assigned, it is implied as undefined.

[alanorozco]

@micajuine-ho might have made this change due to the type issue i mentioned above.