Add OnContextInit System Event

[christianseel]

What does it do?

Adds a new core system event OnContextInit, invoked after each successful context initialization (includes those when switchContext() is called.

Why is it needed?

We are having a project where we need to populate some data, that is context specific. To do so, we added a plugin to run each time a (new) context is initialized. Previously this was only possible via the event OnMODXInit, however this is only fired after the very first initialization of MODX. If you programmatically call switchContext() to init another context, there was no existing event to hook into.

How to test

Add a plugin with a log line and activate the new system event OnContextInit. Log gets printed during modx initialization and when you programmatically call switchContext() with another context key.

Related issue(s)/PR(s)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 21.59%. Comparing base (1b01af2) to head (0bfdc96).

Additional details and impacted files

@@             Coverage Diff              @@
##                3.x   #16552      +/-   ##
============================================
+ Coverage     21.57%   21.59%   +0.01%     
  Complexity    10577    10577              
============================================
  Files           561      561              
  Lines         31975    31982       +7     
============================================
+ Hits           6898     6905       +7     
  Misses        25077    25077              

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