Multiple tenant/connections and Xero strategies

Yes, I haven't designed it to be multi-tenant since I am usually working with only one store. It makes sense as a feature.

Would need to add tenant to the resolver to resolve which strategy to use.

Yeah, Xero API module would need an update first without bypassing it.

The closest I got was trying XeroQuery::addHeader but it didn't appear to set the xero-tenant-id request header in xero client.

I need to support multiple tenants for a current project. If possible, I will try and submit some patches sometime, thanks again.