Create a v2 equivalent for /admin/info/key

The /admin/info/key API has no equivalent in the v2 paradigm. We should close this gap, ideally with an API similar to that below:

• Get Public Key
  • Current v1 API: GET /solr/admin/info/key
  • Proposed v2 API: GET /api/node/key
  • Spreadsheet Ref: Tab 7, Line 18

"Spreadsheet References" are to the Google Sheet available here, which includes additional information about each API, such as the hosting RequestHandler.