Add BookingProject, ContactDetailsGroup and some other small features

[Blumenkind111]

I added the BookingProject to python-easyverein.

Also rewrote the url construction, that it may parse special letters like "&" correctly + some minor fixes

If needed i can update the docs, but cannot promise, i will get to it in time :)

[Blumenkind111]

I also noted, that invNumber has not to be required for InvoiceCreate. That makes it possible to let easyverein calculate the next invoice number. So I implemented that

[waza-ari]

Hey @Blumenkind111 , my apologies for coming back so late. I've finally found the time to go through the PR. First of all, much appreciate your time and commitment adding those two endpoints!

Added a few comments in the review, only one somewhat "major". Would appreciate an update and rebase onto main, although there should be no breaking changes.

Also, let's update the docs folder for those new endpoints. Check the existing ones, most of that is auto-generated so it should be quite quick. Happy to merge afterwards!

[Blumenkind111]

Hi, thanks for the thorough feedback 👍 I included the proposed changes and in the meantime added some more features. I also deleted everything in my branch while trying to update it. Not too familiar with git 😵

I am happy to do more changes if the code does not meet the project standard!

I fixed all comments. Also did some actualization of the docs. The autogenerated part has been done by my bot, because i did not know how ... Together we also updated some outdated parts in the readme and global docs files.

[waza-ari]

Thanks a lot again for this PR! I went through it, ran the tests suite against our demo account and found a few things. They were small, so rather than send everything back I rebased the branch and pushed the fixes on top myself. As a summary, here is what I changed.

Review findings

• The booking receiver still has to be required. test_create_booking_minimal failed, and a manual check confirmed that the API rejects a booking with no receiver or an empty one ("Das folgende Feld muss ausgefüllt sein"). The "" default also never reached the API, because the client only sends fields that were set explicitly. I put receiver back on the required list for BookingCreate and restored it in the test.
• The TODO about methodOfPayment returning a string like "Lastschrift" I could not confirm. I checked every contact in the demo account and also created one with direct debit, and the API returned a number from 0 to 4 every time. int is the right type, so I removed the TODO.
• The budget filters on booking projects only accepted positive whole numbers, so budget__gt=0 or budget__lt=1500.5 were rejected. They're floats now, like the budget field itself.
• Switching get() to exclude_none was the right call, but get_all() still used exclude_defaults, so the same filter could give different results from the two methods. Both build their filters the same way now.
• The new # type: ignore comments in the bulk methods and in member_custom_field.py were hiding type mismatches. With main's client typing and a correct type for the selected-options payload they aren't needed, so I removed them.

All 61 tests pass against the demo account, and lint and mypy are clean. Thanks again for the work you put into this!