feat(nest-iap): use configurable module builder to have forRoot{,Async} methods

[tafaust]

Why this PR?

I wanted to use this library in a production grade setting with ConfigService and Joi for config validation. With the forRoot method of the existing solution, I have no chance to pass the config from my ConfigService to the IAPModule.forRoot(... here be values from config service...).

The configurable module builder creates the forRoot and forRootAsync methods for us and we have to do less tedious work ourselves in the same go.

[tafaust]

@jeremybarbet ping

[franz-geiser]

UP