Authorization on items with a big number of collections

Hello,

Thank you for all the work you put into this!

I am using eoAPI and need to provide fine-grained authorization on collections and items.

Using the `filter` expression seems perfectly valid, and I was able to get it working by specifying a collection filter and applying the same logic as in the example from the [readme](https://github.com/developmentseed/stac-auth-proxy/?tab=readme-ov-file#complex-filter-generator). I simply query the proxy's `/collections` endpoint to get the list of allowed collections.

However, this approach won't scale well as the number of collections increases in the catalog because of the URL size limit when using GET requests. What about having the proxy requesting the backend using POST method for item-search when the backend supports it?

But also, what if I have something like 10,000 collections in my catalog? The CQL2 request will become bloated with all the collections listed. I was wondering if this is a scenario you've already encountered, or if you have any thoughts on potential approach?