Inherited source operators support for ConversionOperatorMapper

[blemasle]

This PR adds support for inherited operators conversion from the source type. Conversion operators on the destination type do not apply here (the operator needs to be about the enclosing type).

I used a modified value of TypeExtensions.StaticFlags in the same idea as this line to match already existing code.

For instance with these changes, such use case works out of the box.

public class IntValue
{
    public IntValue(int value)
    {
        Value = value;
    }

    public int Value { get; }

    public static explicit operator int(IntValue intValue)
        => intValue.Value;
}

public class SomethingId : IntValue
{
    public SomethingId(int value)
        : base(value)
    { }
}

var mapper = new MapperConfiguration(_ => {}).CreateMapper();
mapper.Map<int>(new SomethingId(3)); // works without further configuration

See #3722

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