Add a direct export to ConfigModuleOptions interface and other interfaces

[YarivGilad]

I'm submitting a...

[ ] Regression 
[ ] Bug report
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

Current behavior

At the moment, that is the only way I was able to import this type:

import { ConfigModuleOptions } from '@nestjs/config/dist/interfaces/config-module-options.interface';

const options: ConfigModuleOptions = { envFilePath: '...',  cache:true,  ... }

Expected behavior

import { ConfigModuleOptions } from '@nestjs/config';

const options: ConfigModuleOptions = { envFilePath: '...',  cache:true,  ... }

Environment

"@nestjs/common": "^7.5.1", 
"@nestjs/config": "^0.6.2",

[jmcdo29]

Would you like to create the PR for this?

[kamilmysliwiec]

Added in 0.6.3