add env config to set core with FullCallerEncoder

[zenzenless]

Sometimes we need to view the full file path, so i add an environment configuration item to configure

// full: /full/path/to/package/file:line format.
// short: package/file:line format, trimming all but the final directory from the full path.
envLoggingCallerFormat = "GOLOG_LOG_CALLER_FORMAT" // possible values: full|short