PDI-1516: Add pingone_custom_domain (Resource) Export

[erikostien-pingidentity]

• Fix bug where export environment ID was not being bound to viper configuration properly.
• Remove unneeded parameters from initApiClient()
• Updated --pingone-export-environment-id help message to include the available environment variable.
• Added the ImportBlock Sanitize() function to make sure the "to" field is lowercase, without special characters, and spaces replaced with underscores.
• Remove "Resource" from creation methods for each exportable resource following effective go conventions.
• Do not create import.tf file for empty exported resources.