fixed following issues:

Created by: vthiruveedula

1. Generation of apis for callbacks
   Modified DefaultGenerator.java file to generate callback APIs
2. Api.mustache file changes
       -  Request and Response models as params
       -  changed template to compatible with free5gc apis
 3. Generating models without AnyOf tag
 4. Generating  model go file for RequestBody with the name appended as ‘Request’
 5. Generating  model go file for ResponseBody with the name appended as ‘Response’
 6. Model is not getting generating if any of enum values conflicted with other enum value of enum type.
 7. If the components schema has ‘Allof’ type as string, refered schema is not setting it as string

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