Add forRootAsync for dynamic configuration (ConfigService)

[Jordi Escudero Subirana]

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

Not implemented

Expected behavior

Same behavior than TypeOrmModule.forRootAsync(...)

[Kamil Mysliwiec]

Added in 2.0.0