feat: allow integer vehicle IDs

[bklebe]

Summary of changes

Asana Ticket: allow integer vehicle IDs in concentrate in case they get passed through

Logs a warning if an integer vehicle ID is encountered and stringifies it with Integer.to_string/1.

[github-actions]

Coverage of commit a95dbc6

Summary coverage rate:
  lines......: 94.5% (1243 of 1316 lines)
  functions..: 83.1% (404 of 486 functions)
  branches...: no data found

Files changed coverage rate:
                                                                    |Lines       |Functions  |Branches    
  Filename                                                          |Rate     Num|Rate    Num|Rate     Num
  ========================================================================================================
  lib/concentrate/parser/gtfs_realtime_enhanced.ex                  |97.6%     82| 100%    17|    -      0

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[paulswartz]

question: should this also handle the id field in the VehiclePosition struct?