Snowplow v2

[colmsnowplow]

Proposed changes for a PR to ampproject/amphtml.

• Added custom context functionality (to be manually entered as SD-JSONs, with a trailing comma)
• Added default amp_id and amp_page_view contexts - sent with every event (related Iglu Central PR).
• Added custom events.
• Added page pings.
• Added parsing of js-tracker cross-domain linker url value - if found, value is retained for the lifetime of amp-set cookie.
• Added amp linker passing of amp client id to outgoing links.
• Additional standard tracker protocol fields added.
• ampdocurl used for url.
• Domain Userid setting removed (inconsistent with js tracker).

[colmsnowplow]

Closing, PR to ampproject/amphtml forthcoming.

[colmsnowplow]

https://github.com/ampproject/amphtml/pull/26059