Az.CosmosDB should support Pipeline-input

## Description of the new feature

Right now the commands don't support Pipeline-Input.
For example Get-AzCosmosDbAccount requires the Resourcegroup as String and doesn't allow an RG-Object to be passed in.

Also the command should just list all CosmosDBAccounts in the subscription when no Name or RG is given... It's a bit tedious to have to use Get-AzResource to get the CosmosDBs and then use the Get-AzCosmosDbAccount-Cmdlet to get the proper Object to work with...

## Proposed implementation details (optional)

My C#-Skills are not good enough yet and I hope someone else can do that before I learned all those things...