DEVX-7400: Add Subaccounts API

[superchilled]

This PR adds functionality for the Subaccounts API to the SDK. Specifically, it does the following:

• Update the Client class to add a subaccounts method (which returns a Subaccounts object)
• Defines a Subaccounts object with the following methods:
  • list
  • find
  • create
  • update
  • list_credit_transfers
  • transfer_credit
  • list_balance_transfers
  • transfer_balance
  • transfer_number
• Additionally it:
  • Adds ListResponse classes for list, list_credit_transfers, and list_balance_transfers
  • Adds unit tests for the above functionality

[codecov-commenter]

Codecov Report

Merging #275 (0db8ba1) into dev (bfa33dd) will increase coverage by 0.05%. The diff coverage is 100.00%.

:exclamation: Current head 0db8ba1 differs from pull request most recent head 3e70a30. Consider uploading reports for the commit 3e70a30 to get more accurate results

@@            Coverage Diff             @@
##              dev     #275      +/-   ##
==========================================
+ Coverage   97.96%   98.01%   +0.05%     
==========================================
  Files          80       84       +4     
  Lines        1622     1666      +44     
==========================================
+ Hits         1589     1633      +44     
  Misses         33       33              

Impacted Files	Coverage Δ
lib/vonage/namespace.rb	100.00% <ø> (ø)
lib/vonage/client.rb	100.00% <100.00%> (ø)
lib/vonage/subaccounts.rb	100.00% <100.00%> (ø)
...age/subaccounts/balance_transfers/list_response.rb	100.00% <100.00%> (ø)
...nage/subaccounts/credit_transfers/list_response.rb	100.00% <100.00%> (ø)
lib/vonage/subaccounts/list_response.rb	100.00% <100.00%> (ø)

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[superchilled]

@SMadani I've made the requested changes :)