Azure.Search: Clean up JSON parsing

[tg-msft]

Ahson left a lot of super valuable feedback on #10568 about the best patterns for using System.Text.Json including:

• Avoid allocating so many extra strings for property name checks
• Don't assert on token types when calling Get* methods
• Whether or not we can use converters for SearchDocument
• Encoding our constant values
• Using a JsonConverter for null arrays