add POST - /collections collection-search

[vincentsarago]

in addition to the collection-search extension work done in #736, this PR adds a POST - /collections endpoint in another extension CollectionSearchPostExtension

[m-mohr]

How does this work in combination with Collection Transactions?

[vincentsarago]

@m-mohr it shouldn't be used together, which is why I've created 2 different extension (with and without the POST endpoint)

It will be really weird to have a POST - /collections being able to handle both the transaction and the search IMO.

Maybe we can investigate how to do it in another iteration but for now I believe it's fine having both extensions adding their own POST - /collections endpoint.

In term of code, it will be really complex (nor impossible) to make them work together because they are both extensions so we can't make them depends on each others

[m-mohr]

Ah, I didn't realize that. Thanks for the clarification, sounds good to me.