Add MVP rule for AWS::Events::Rule ScheduleExpression

[fatbasstard]

Issue #816, if available:

This PR adds the initial (mvp) version of the rule to validate the ScheduleExpression of an AWS::Events::Rule.

These expressions can become pretty complex, and to prevent a bing-bang release of a complex rule that might break because of "reasons" (incorrect documentation, missed use-cases etc) this is a first version that adds:

• The Rule to validate the ScheduleExpression
• Test templates ("good" templates based on the given examples)

The rule now checks:

• Only specific values (no intrinsic functions)
• Checks for cron() or rate()
• Validates the basic format of these 2 configuration

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

Merging #1028 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1028      +/-   ##
==========================================
+ Coverage   86.89%   86.97%   +0.07%     
==========================================
  Files         133      134       +1     
  Lines        7892     7938      +46     
  Branches     1947     1954       +7     
==========================================
+ Hits         6858     6904      +46     
  Misses        640      640              
  Partials      394      394

Impacted Files	Coverage Δ
...t/rules/resources/events/RuleScheduleExpression.py	100% <100%> (ø)

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[fatbasstard]

Can be merged