Add attributes for Format derive to use Debug2Format on specific fields

[mattico]

I often have structs or enums where most fields impl Format, but a few third-party types do not. It is a pain to have to manually impl Format for the whole type when only one field doesn't work. I added an attribute that lets you override specific fields to use Debug2Format or Display2Format like so:

#[derive(defmt::Format)]
pub enum Error {
    Serial(serial::Error),
    Timeout(us_timer::TimeoutError),
    Serde(#[defmt(Debug2Format)] serde_json::Error),
    ResponseTooLarge,
}

[Urhengulas]

Looks really good! Thank you @mattico!

[Urhengulas]

Since I am not super proficient with macros I'd like @jonas-schievink to take a quick look before merging.

[jonas-schievink]

Great to see this, thanks!

bors r+

[bors]

Build succeeded:

• ci

[mattico]

I suppose these didn't need to be if lets

[Urhengulas]

True, either normal if-else or match might be better suited.