Add event kit API

[lloeki]

What does this PR do?

Implement event sugar API for users to emit either login events or custom business events to be tracked.

Motivation

These events are a key feature that Sqreen had and enabled higher level analysis and partially to fully automated security response.

Additional Notes

Just as with Datadog::Kit::Identity#set_user this looks like convenience syntactic sugar. Nonetheless, just as with set_user, wrapping the tag keys allows for these keys to evolve, or even the transport mechanism to change.

In addition, blocking may be handled and triggered, which is probably a feature we don't want in set_tag!

How to test the change?

There are a bunch of specs added.

[codecov-commenter]

Codecov Report

Merging #2512 (e4eb66a) into master (1ca13b5) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2512      +/-   ##
==========================================
- Coverage   98.32%   98.32%   -0.01%     
==========================================
  Files        1124     1126       +2     
  Lines       60660    60725      +65     
==========================================
+ Hits        59644    59707      +63     
- Misses       1016     1018       +2     

Impacted Files	Coverage Δ
lib/datadog/kit/events.rb	100.00% <100.00%> (ø)
spec/datadog/kit/events_spec.rb	100.00% <100.00%> (ø)
lib/datadog/core/workers/polling.rb	96.55% <0.00%> (-3.45%)	⬇️
...dog/profiling/collectors/cpu_and_wall_time_spec.rb	97.48% <0.00%> (-0.42%)	⬇️
spec/datadog/profiling/native_extension_spec.rb	98.74% <0.00%> (+0.62%)	⬆️

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