Function parse_field_selection 

pub fn parse_field_selection(query: &str) -> Result<FieldSelection>

Parse GraphQL query to extract field selection for _entities.

Example query:

query {
  _entities(representations: [...]) {
    __typename
    id
    name
    email
  }
}

Returns: FieldSelection { fields: [“__typename”, “id”, “name”, “email”] }

Errors

Returns error if query parsing fails.