Add More Asset Management Commands

[AndrewPla]

I need to add a few new commands for dealing with assets. In particular, there is no method for retrieving/updating grid fields from assets.

[AndrewPla]

Also need to add Remove-TdGridFieldAsset

[mabster]

Would love to see a -Filter parameter on Get-TdAsset. Would allow us to quickly get a list of assets by serial number, amount of RAM etc (since it works with custom fields).