feat: add customer and request endpoints for service desk api

[iamjem]

Information that is useful here:

• The What: This adds endpoints for customers and requests in the service desk api.
• The Why: Because these endpoints are missing (and I need them in my project).
• Type of change: New feature.
• Breaking change: This change should be backwards compatible.
• Related to an issue: Not that I'm aware of.
• Jira Version + Type: Cloud

Example:

Tests exist for all new endpoints, which is probably the best source of sample code.

Checklist

• Unit or Integration tests added
  • Good Path
  • Error Path
• Commits follow conventions described here:
  • Conventional Commits 1.0.0
  • The seven rules of a great Git commit message
• Commits are squashed such that
  • There is 1 commit per isolated change
• I've not made extraneous commits/changes that are unrelated to my change.

[frzifus]

Is there a reason why we expose this method without a context?

[iamjem]

There's a CustomerService.CreateWithContext which is the context version of the method. I followed this pattern from other service endpoints in the package.

[iamjem]

Thank you for the review! I believe that I've addressed your comments, please let me know if there's any other changes needed.

[benjivesterby]

Please update as requested by @james-cast since the service desk id is supposed to be a string.

[iamjem]

I have updated serviceDeskID int to serviceDeskID interface{} for the remaining customer methods, and added test coverage.

[benjivesterby]

LGTM