feat: Support serde(flatten) in IntoParams derive

[esavier]

fixes: #1531

Expand flattened fields that implement IntoParams into the parent parameter list instead of generating a parameter for the flattened field itself.

[esavier]

i hope i did this correctly, please add your comments if we think this should be done differently

[CommanderStorm]

Can you compare your PR with the other open PR implementing this?
Which one is better and why?

• https://github.com/juhaku/utoipa/pull/1426/changes

Your code does not have the weird utoipa-gen/tests/into_params_test.rs but your code is also a lot shorter.
IF there is a difference, make sure to also add a testcase for that difference, as that would mean that the tests are not 100% 😉?

[esavier]

Yup, i will do what i can.
at a glance both options are okay. I do not think the other PR is necessarily disadventagious, but i am new here, i will check how the rest of the code looks like and what we can do. Please give me time until tomorrow.