add support for messageId

[WaleedAshraf]

Breaking:

• "should" replaced with "must" in the error messages, as per in AJV

New / Improvements:

• Support AsyncAPI schema v2.4.0
• Added validateByMessageId() method, which only requires key and value. To use this method, your AsyncAPI schema should be >= 2.4.0. messageId should be defined in schema as per AsyncAPI 2.4.0 schema definition.
• msgIdentifier option is only required if you use .validate() method.
• You can use both validateByMessageId() and validate() methods if your AsyncAPI schema version is >= 2.4.0
• Start using ajv-formats for custom formats.

closes #25, closes #51

[codecov]

Codecov Report

Merging #52 (87e2013) into master (6c4e01c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #52   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            6         6           
  Lines           98       117   +19     
  Branches        19        30   +11     
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+ Hits            98       117   +19