Closes #17653 Add function to trim whitespaces in export templates via jinja environment settings

[renatoalmeidaoliveira]

Fixes: #17653 Add function to trim whitespaces in export templates via jinja environment settings

• Creates RenderMixin model with fields: template_code, environment_params, mime_type, file_name, file_extension, as_attachment
• Add RenderMixin to ExportTemplate and ConfigTemplate, removing the repeated fields
• Unify the export logic inside RenderMixin using ExportTemplate as reference for exports, i.e. always using template_code instead of using data_file when exists, because template_code should be equals to data_file after sync and it gives the end user more control of what template is in use.

Extra Comments:

• Allowing the user to setup file_name and file_extension to ConfigTemplate may give the user more flexibility in ZTP use cases using NetBox as source of python ZTP configuration files,

[jeremystretch]

Make sure you're completing the checklist for extending models each time you add a new field. The REST API serializers, GraphQL types, and several forms also need to be updated to reflect the new fields.