Make the HostHooks shareable between app and context

[hansl]

Previously it was very difficult to keep a stateful HostHooks, since it needed to be a static reference. Now it can be shared. This is particularly useful for mocking hooks for testing.

This also follows the pattern for the job executor and the module loader.

I did not see any impact on performance.

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 4 lines in your changes missing coverage. Please review.

Project coverage is 53.55%. Comparing base (6ddc2b4) to head (4ec5884).
Report is 355 commits behind head on main.

Files with missing lines	Patch %	Lines
core/engine/src/context/mod.rs	66.66%	2 Missing ⚠️
core/engine/src/builtins/date/mod.rs	97.14%	1 Missing ⚠️
core/engine/src/object/builtins/jsdate.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4141      +/-   ##
==========================================
+ Coverage   47.24%   53.55%   +6.31%     
==========================================
  Files         476      487      +11     
  Lines       46892    48874    +1982     
==========================================
+ Hits        22154    26176    +4022     
+ Misses      24738    22698    -2040     

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[jedel1043]

IIRC we had Rc<HostHooks> before, but it was determined to be unnecessary since the hooks are not supposed to carry additional data, opting for HostDefined if some special data was needed.

I think it's fine to make it possible to add special data to the hooks, but I'm slightly worried that some subtle bugs could be triggered because the hooks are used at realm creation time.

[jedel1043]

Preemptively approving since this is immediately useful, but it would be good to make a survey on how the host hooks are defined on other engines to be sure that we're not introducing bugs by doing this.