VAPI-3162 Add <Refer> BXML verb support

[nkuma246]

This pull request adds support for the new Refer BXML verb to the Bandwidth SDK, enabling users to generate and manipulate Refer elements in their BXML call flows. The implementation includes the new verb class, updates to the gem template, and comprehensive unit tests for the new functionality.

New BXML verb support:

• Added a new Refer verb class (Refer) in lib/bandwidth-sdk/models/bxml/verbs/refer.rb with support for nested SipUri elements and relevant attributes such as referCompleteUrl, referCompleteMethod, and tag.
• Updated the gem template (custom_templates/gem.mustache) to require the new refer verb, ensuring it is included in the SDK build.

Testing:

• Added unit tests for the new Refer verb in spec/unit/models/bxml/verbs/refer_spec.rb, covering initialization, attribute updates, BXML generation, and handling of nested SipUri elements.

[bwappsec]

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