feat: add async support

[dojje]

Summary

Adds AsyncBlocketAPI for async/await support. All search parameter logic is extracted into a shared _params.py module so the sync and async wrappers follow DRY — zero duplicated URL/param construction.

Architecture

_params.py          ← shared: build_search_params / build_car_params / build_boat_params / build_mc_params
    ↙         ↘
blocket.py    async_blocket.py   ← thin wrappers: only HTTP call differs

• blocket_api/_params.py — pure functions that return (url, httpx_params) for every endpoint
• blocket_api/blocket.py — refactored to delegate to _params.py (no more inline QueryParam logic)
• blocket_api/async_blocket.py — new AsyncBlocketAPI class using httpx.AsyncClient, supports async with
• tests/async_requests.py — 11 async tests mirroring the existing sync suite

Usage

from blocket_api import AsyncBlocketAPI

async with AsyncBlocketAPI() as api:
    results = await api.search("iPhone")
    cars = await api.search_car(models=[CarModel.VOLVO], locations=[Location.STOCKHOLM])

Verification

• 22/22 tests pass (11 sync + 11 async)
• mypy reports no type errors

[dojje]

For context: this follows up on PR #11 from last year.

[dunderrrrrr]

Thanks for your PR, this will be a great improvement.

I need to have a more thorrow look at this before merging but overall it looks good!

One thing though, could you maybe split the commits into multiple ones? I think the query param functions being moved to _params.py can be a separate commit. It'll be easier to review 🙂

[dojje]

Split into two commits as you suggested. Params function only returns params and not url. Constants for each function.

[dunderrrrrr]

I have tested the code and it looks fine to me! There should not be any breaking changes here from what I can tell.

If you want to, feel free to add another commit updating the README, should be enough with just one example there I think. I can make the appropriate changes to the docs later 🙁

Thanks for your contribution! This will be a significant improvement 🚀

[dojje]

I've changed the readme now