DEVX-6418: Adding Voice NCCO Pay action

[superchilled]

Reason for this PR

The NCCO reference has added an action for Pay, which therefore needed to be implemented in the Ruby SDK.

What this PR does

• Defines an NCCO Pay action under Voice::Actions
• Adds tests for this new action

[codecov-commenter]

Codecov Report

Merging #244 (59a7fd2) into dev (0c4059b) will increase coverage by 0.02%.
The diff coverage is 98.24%.

❗ Current head 59a7fd2 differs from pull request most recent head 442b079. Consider uploading reports for the commit 442b079 to get more accurate results

@@            Coverage Diff             @@
##              dev     #244      +/-   ##
==========================================
+ Coverage   97.71%   97.73%   +0.02%     
==========================================
  Files          70       71       +1     
  Lines        1400     1457      +57     
==========================================
+ Hits         1368     1424      +56     
- Misses         32       33       +1     

Impacted Files	Coverage Δ
lib/vonage/voice/actions/pay.rb	98.24% <98.24%> (ø)

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