Support form encoded POST requests

[fengelniederhammer]

Support accepting form encoded requests with the content type application/x-www-form-urlencoded. This improves browser support, because a browser can simply submit a form like this to get a valid LAPIS call:

<form action="my.lapis/sample/aggregated" method="POST">
    <input type="text" name="country" />
    <input type="submit" value="Submit" />
</form>

For GET methods, this is already supported by LAPIS, since the form data just ends up in the query string. URLs have a length limited, so this doesn't work for large requests (e.g. someone requesting data for 1000 primary keys).

• This should be supported for every endpoint
• It should support setting the dataFormat to return the data as JSON, CSV or TSV.

Implementation-wise we could implement it in two different ways:

1. accept form value query that is expected to contain a JSON that translates to a valid POST request to the respective endpoint.
2. accept all fields separately in the URL encoded form format.

We could also support both. Let's see how well Spring supports form url encoded requests and what is easiest to implement.