Support schema defaults for new events.

[f2prateek]

If an event is not present in the tracking plan, we'll now lookup the schema defaults for the project.

If the event is disabled in the schema default, it will only be sent to the Segment integration (i.e. api.segment.io).

If the event is enabled in the schema default, it will be sent to all integrations.

Ref: https://paper.dropbox.com/doc/Schema-Client-Side-Defaults-DufdS8Ej43mnFXvvMqm1b

[codecov-io]

Codecov Report

Merging #552 into master will increase coverage by 0.02%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master     #552      +/-   ##
==========================================
+ Coverage   83.22%   83.24%   +0.02%     
==========================================
  Files          30       30              
  Lines        2444     2453       +9     
  Branches      278      281       +3     
==========================================
+ Hits         2034     2042       +8     
- Misses        327      328       +1     
  Partials       83       83

Impacted Files	Coverage Δ
...va/com/segment/analytics/IntegrationOperation.java	89.52% <80%> (-0.06%)	⬇️

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[danielstjules]

LGTM :)

[f2prateek]

Made some test updates to get better coverage.