Add projectCount to Round schema to enable filtering and sorting

[carlbarrdahl]

It would be great if it was possible to do queries such as:

• Return rounds with projectCount > 0
• Order the results to display rounds with the most (and least) projects first

It could be done by:

• Adding projectCount to the schema
• Incrementing the value here: https://github.com/allo-protocol/graph/blob/main/round/src/round/implementation.ts#L112

Here's what the query would look like

rounds(where: { projectCount_gt: 0 }, orderBy: projectCount) {
  ...
}