Hi,
This is a first attempt at implementing extensions within the utoipa::path macro. Read the PR in conjunction with the issue #1228 .

The extensions is implemented for the following parts of the utoipa::path macro:

• At operation level;
• Operation > Params > ParamItem
• Operation > Responses
• Operation > Responses > ContentType
• Operation > RequestBody
• Operation > RequestBody > ContentType

The implementation uses the form (as discussed in the issue):

("x-my-extension" = json!({"key": "value"}))

The parsing and building of the token stream is done in a struct Extensions - which I located under components/features/attributes. I decided to split it into its own file that is loaded in attributes.rs as attributes.rs is already quite long. But not sure (a) if it makes sense as a child of attributes and (b) if ok to split into its own separate file.

Also, I used use super::*; at the top of the extensions.rs. Happy to modify to just what is needed if this is the recommended approach.

What is included:

• Tests [for the happy path]
• An example. The example is showing both macro and Modify trait approaches [this was a way for me to check which one were replaceable] - but having both there may be too much.

Closes #1228 Closes #1255