Add new fields to PullRequests query

Closes #170
- Added new fields (specified in #170) to the `PullRequests` GraphQL query.
- Extended `GitHubPullRequests` to store the additional data.
- Implemented support for nested types like labels, comments, reviews,
  and commits.
- Derived `Serialize`/`Deserialize` for new types to enable database
  storage.

Author: jadenow

CHANGELOG.md

@@ -15,6 +15,8 @@ this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
   `assignee`, `author`, `repo`(repository name), `begin` and `end` (creation
   date range). The query returns the `openIssueCount` field, indicating the
   number of open issues.
+- Added new fields to the `PullRequests` GraphQL query and corresponding fields to
+  the `graphql::pull_request::PullRequest` struct.
 
 ### Changed