Add more AWS completers

[mtboren]

There are a couple of ripe targets for some argument completion: Get-AWSCmdletName and Get-AWSService. Some suggested params for which to add argument completers:

Cmdlet	ParameterName	Info
Get-AWSCmdletName	ApiOperation	Likely get the (nearly complete?) list of available API operations from Get-AWSCmdletName itself
Get-AWSCmdletName, Get-AWSService	Service	Can get the list of AWS services from Get-AWSService; should add ServiceName in the completer tooltip