Pass cookie values to amp-analytics

### Description

I'd like to pass a cookie value to our in house analytics platform.

One thought was just adding a new substitution variable that can be used in the url. This would be similar to how `clientId` works at the moment.

So ideally, we could do something like:

url: `https://inhousetracking.com/tracking?${ampCookie('cookieName')}`

### Alternatives Considered

There is a way to use linker and query params variables? But that's not really what we want

### Additional Context

_No response_

Start by locating amp-analytics and the existing clientId substitution-variable handling; the issue proposes a similar URL variable for a named cookie. Confirm the intended cookie-name and value syntax with maintainers, then add coverage for cookie-backed URL substitution and verify existing analytics substitutions remain unchanged.

Written by the indexing model from the issue text.