Add weights summary to issues GraphQL

Release notes

Problem to solve

For #267194 we need an ability to fetch total weights of issues for given month through GraphQL.

Intended users

Any API user

Proposal

Create additional GraphQL field for Issues connection which gives summary about issues under selection. E.g.

issues {
  weight
}

Documentation

• New field should be automatically mentioned as part of general GraphQL documentation.

Links / references