List Offerings per Community missing as rpc call

[anizeani]

In #87 the rpcs 1. get businesses per community and 2. get offerings per business are added,
The rpc call to get offerings per community is missing, I guess (it doesn't appear on polkadot-js and I didn't find it in the rust code:
dcae0d7

[clangenb]

This is in fact something we can discuss. Currently, you need to make a two layered approach:

1. Get all businesses per community
2. Get all these businesses' offerings.

[brenzi]

We can keep this as a possible enhancement, but I see no reason to change it now as everything is possible as is