support commas in data_attrs query param

[annashamray]

Fixes #472

We use comma as a separator between data_attrs values
If we want to support commas inside values we can consider several options:

1. Remove several value groups, separated by commas, so comma won't be a reserved character here
   Before: ?data_attrs=naam__icontains__anna,naam__icontains__Advies
   After:?data_attrs=naam__icontains__anna&data_attrs=naam__icontains__Advies, support

   This is my personal favorite option, but it will break current API

2. Change separator from comma to other character, for example to |.
   Before: ?data_attrs=naam__icontains__anna,naam__icontains__Advies
   After: ?data_attrs=naam__icontains__anna|naam__icontains__Advies, support

   Also a nice option, but also breaks current API

3. Escape comma in the value part or use other char instead of comma in the value
   Here comma in the value is replaced with \,
   Before: ?data_attrs=naam__icontains__anna,naam__icontains__Advies
   After: ?data_attrs=naam__icontains__anna,naam__icontains__Advies\, support

   This options is implemented in the PR, since it doesn't break the API

4. Use some regex magic to define when comma is used as separator and when it's used as a part of value.
   Let's not do it, since it's very tricky and unclear what happens

What do you think is the best way to implement it?
Should we introduce the breaking change? The next big release already contains a breaking change, since we remove v1 endpoints

@joeribekker @stevenbal @SonnyBA @Coperh let's discuss it

[joeribekker]

But, when we discussed this, you said the ," is still seen as a comma and parsed early on?

[annashamray]

@joeribekker Yes, but then I realized that we still can treat \, as not comma. Please take a look at the implementation

[stevenbal]

Minor comments, looks good overall 👍

[stevenbal]

Nitpicky, but ideally explode: true would be specified here to indicate that it should be used like ?data_attr=...&data_attr=... (see https://swagger.io/docs/specification/v3_0/serialization/#query-parameters)

For example: https://github.com/open-formulieren/open-forms/blob/3a702080d35c0670588cc69b8e49445e6224989c/src/openforms/appointments/api/views.py#L64

Although looking at it, apparently explode: true is considered the default by OAS3, so I'm not sure if our schema is correct in that case 🤔. Might be something to investigate in another issue

[annashamray]

I've added it to the OAS and created issue to investigate all filters #498