Adds Issues 2.0 custom field values to each issue returned by the
list_issues GraphQL query, exposed on MinimalIssue as field_values:
[{field, value}]. Filtering by field is a separate concern (needs the
GraphQL IssueFilters input updated upstream) and is not included here.
shurcooL/graphql's response decoder walks every inline fragment of a
union regardless of __typename, so IssueFieldNumberValue.value is
aliased to valueNumber to avoid a Float-vs-String type clash when the
runtime variant is, e.g., a SingleSelectValue.
Kelsey Myers
b57cd61907