Create a v2 equivalent for /admin/cores BACKUPCORE+RESTORECORE

The /admin/cores?action=BACKUPCORE and RESTORECORE APIs have no equivalent in the v2 paradigm. We should close these gaps, with v2 APIs like the ones below:

• Create a core-level backup
  • Current v1 API: GET /solr/admin/cores?action=BACKUPCORE&core=coreName
  • Proposed v2 API: POST /api/cores/backups/backupName {...}
  • Spreadsheet Ref: Tab 1, Line 88
• Restore from a backup to a core
  • Current v1 API: GET /solr/admin/cores?action=RESTORECORE&core=coreName
  • POST /api/cores/coreName/commands/restore
  • Spreadsheet Ref: Tab 6, Line 39

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