feat: Add support for URL custom property value type

[IrvingMg]

Since ValueType is just a string field with no validation, I only updated the documentation comment and added a test. Let me know if I'm missing something.

Fixes #3878

[stevehipwell]

It might make sense to add type for ValueType?

[IrvingMg]

It might make sense to add type for ValueType?

I think it would make sense. However, as I see it, the pattern in the library is to use similar enum-like fields as plain strings with documentation comments, such as:

go-github/github/repos.go

Lines 75 to 78 in 07ddcd9

	SquashMergeCommitTitle *string `json:"squash_merge_commit_title,omitempty"` // Can be one of: "PR_TITLE", "COMMIT_OR_PR_TITLE"
	SquashMergeCommitMessage *string `json:"squash_merge_commit_message,omitempty"` // Can be one of: "PR_BODY", "COMMIT_MESSAGES", "BLANK"
	MergeCommitTitle *string `json:"merge_commit_title,omitempty"` // Can be one of: "PR_TITLE", "MERGE_MESSAGE"
	MergeCommitMessage *string `json:"merge_commit_message,omitempty"` // Can be one of: "PR_BODY", "PR_TITLE", "BLANK"

But if you’re okay with adding types, I could add constants like:

const (
      PropertyValueTypeString      = "string"
      PropertyValueTypeSingleSelect = "single_select"
      PropertyValueTypeMultiSelect  = "multi_select"
      PropertyValueTypeTrueFalse    = "true_false"
      PropertyValueTypeURL          = "url"
  )

And we can decide later if we want to add similar enums for other fields across the library.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.48%. Comparing base (07ddcd9) to head (e19940b).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3879   +/-   ##
=======================================
  Coverage   92.48%   92.48%           
=======================================
  Files         200      200           
  Lines       14564    14564           
=======================================
  Hits        13469    13469           
  Misses        895      895           
  Partials      200      200           

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