Allow extensions_options to accept Option field

[goldmedal]

Which issue does this PR close?

no related issue.

Rationale for this change

We allow users to define the custom config by extensions_options like

    extensions_options! {
        struct TestExtension {
            value: usize, default = 42
        }
    }

However, the Option type isn't allowed. It's hard to handle the null value case.

This PR allows user to define the field like

    extensions_options! {
        struct TestExtension {
            value: usize, default = 42
            option_value: Option<usize>, default = None
        }
    }

What changes are included in this PR?

• Implement ConfigField, which has been implemented for many data types, within the macro.

Are these changes tested?

add unit test.

Are there any user-facing changes?

New behavior for extensions_options macro

[alamb]

Looks good to me - thank you @goldmedal

I double checked and I am pretty sure this PR is backwards compatable / has no API changes. Specifically, no existing code would need to be changed, right?

[alamb]

👍

[goldmedal]

I double checked and I am pretty sure this PR is backwards compatable / has no API changes. Specifically, no existing code would need to be changed, right?

Yes. no existing code needs to be changed.