splitter: Support JSONPath references

[antoineco]

#5 suggested that JSONPath expressions could be used to

• Express the path of the data to split on (e.g. .items for an "items" key located directly at the root)
• Populate CloudEvents attributes (e.g. "user/{.name}" to generate "user/johndoe")

It's not the case right now, but I think it would be a nice to have. We could even reuse that code in several TriggerMesh integrations.