DEVX-6027: Add Messages API v1

[superchilled]

Reason for this PR

The [Messages API]() is now in General Availability (GA), and so we are adding it to all of the supported Vonage server SDKs.

What this PR does

• Defines various classes for creating message objects, a general Message class and then sub-classes for each channel:
  • Messenger
  • MMS
  • SMS
  • Viber
  • WhatsApp
• Defines a Messaging class inheriting from Namespace, which defines a send method for sending messages
• Adds a messaging method to the Client class for creating/ returning Messaging objects.
• Adds unit tests to cover the new functionality.

[codecov-commenter]

Codecov Report

Merging #221 (56169eb) into dev (2860880) will increase coverage by 0.24%. The diff coverage is 100.00%.

:exclamation: Current head 56169eb differs from pull request most recent head b992b5a. Consider uploading reports for the commit b992b5a to get more accurate results

@@            Coverage Diff             @@
##              dev     #221      +/-   ##
==========================================
+ Coverage   97.43%   97.68%   +0.24%     
==========================================
  Files          63       70       +7     
  Lines        1249     1384     +135     
==========================================
+ Hits         1217     1352     +135     
  Misses         32       32              

Impacted Files	Coverage Δ
lib/vonage.rb	91.66% <ø> (ø)
lib/vonage/client.rb	100.00% <100.00%> (ø)
lib/vonage/messaging.rb	100.00% <100.00%> (ø)
lib/vonage/messaging/channels/messenger.rb	100.00% <100.00%> (ø)
lib/vonage/messaging/channels/mms.rb	100.00% <100.00%> (ø)
lib/vonage/messaging/channels/sms.rb	100.00% <100.00%> (ø)
lib/vonage/messaging/channels/viber.rb	100.00% <100.00%> (ø)
lib/vonage/messaging/channels/whats_app.rb	100.00% <100.00%> (ø)
lib/vonage/messaging/message.rb	100.00% <100.00%> (ø)

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