Polymorphic custom property values (fixes deserialization bug)

[colbylwilliams]

Resolves #520

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Before the change?

• Custom Property and Custom Property Value schemas added new value_types and updated the default_value and value properties to allow for null, string, or array. This breaks deserialization of payloads with Custom Property or Custom Property Values. See issue for more details.

After the change?

• Added multi_select, and true_false custom property value types
• Custom Property definition default_value may be null, string, or array of strings
• Custom Property value property may be null, string, or array of strings
• Updated Repository.CustomProperties to be IDictionary<string, object> to support polymorphic values of either string or array

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[github-actions]

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[colbylwilliams]

cc: @JamieMagee