Create Data Center GQL type, query, and resolver

[joneubank]

Detailed Description

• [x] Create DataCenter type in gql schema
• [x] Add dataCenters query
  • [x] Create dataCenters query resolver that will retrieve list of all data centers from ProgramService
  • [x] Add listDataCenters method to the program service HTTP client that will fetch data centers from ProgramService

[daniel-cy-lu]

Zenhub issue, so the PR is here: https://github.com/icgc-argo/platform-api/pull/700