Add support for SERVICE_NOW_APP

[atkinchris]

Description

This adds support for the Service Now Certified App integration, as an alternative to the legacy Service Now integration.

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

Checklist:

Please delete options that are not relevant.

• [x] My commit message follows conventional commits
• [x] My code is formatted to Go standards
• [x] I have performed a self-review of my own code
• [x] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes. Go here for instructions on running tests locally.

How to test this change?

Create a newrelic_notification_destination with type = SERVICE_NOW_APP.

[CLAassistant]

All committers have signed the CLA.

[amaor-newrelic]

@atkinchris Could you add it to listValidNotificationsChannelTypes as well? Basically just like you searched for SERVICE_NOW and added SERVICE_NOW_APP, search for SERVICENOW_INCIDENTS and add SERVICE_NOW_APP Edit: Nevermind! We'll merge this first and then do it! :D

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 36.62%. Comparing base (92361de) to head (2e048bb). Report is 56 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2693      +/-   ##
==========================================
+ Coverage   32.82%   36.62%   +3.80%     
==========================================
  Files          98       98              
  Lines       26884    21809    -5075     
==========================================
- Hits         8824     7988     -836     
+ Misses      17902    13658    -4244     
- Partials      158      163       +5     

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