Fix and add a test for the cookie reader, because the cookie name or value could change (externally)

Problem/Motivation

Exception: API key is not set in the configuration. in Drupal\posthog\CookieReader->getCookieName() (line 60 of /var/www/drupal/web/modules/contrib/posthog/src/CookieReader.php).

on first login after module installation!

Problem 2:
In Cookie Reader we assume a certain name for the cookie we read.

The PH docs say:

By default, we store all this information in both a cookie and localStorage, which means PostHog can identify your users across subdomains. By default, the name of the cookie PostHog sets is ph_
_posthog and it expires after 365 days.

Mind ph_
_posthog
https://git.drupalcode.org/issue/posthog-3586186/-/blob/3586186-identify...

So this should be relatively reliable.

Still we should add a test which does something to create this cookie and then in PHP should check to read it for 3 reasons:
Ensure the functionality works as expected in general
The cookie name is not changed
The cookie value we read is not changed

Both last tests should contain a message that if this fails, probably PostHog changed the name and that should be checked first!

Please link the docs there: https://git.drupalcode.org/issue/posthog-3586186/-/blob/3586186-identify...

I also though if we should move the cookies prefix, suffix and value distinct_id into config variables, but I think that might be prone to errors. Please instead move them to constants in the cookie reader!

Steps to reproduce

Log in in a fresh installation

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes