Resolve Attestation Factory Method vs Factory Class dispute.

[LarryOsterman]

@vhvb1989 points out that factory methods have some functional drawbacks, there is an outstanding discussion with the API review team about the best solution.

THis bug tracks fixing this, and it MUST be fixed before the Attestation SDK GA's.

The not-so-good about factory methods is that they are accessible from class instances as well.
So, you attestation client will also list the client.Create() method just like the class AttestationClient::Create().

What I usually prefer is to create the factory as an additional class with no constructor (real factory). Something like AttestationClientFactory.

Originally posted by @vhvb1989 in #3543 (comment)

[LarryOsterman]

Because the intellisense experience for factory classes is better than the experience for factory methods, settled on a factory class.

Also created Azure/azure-sdk#4349 to add guideline documentation on this pattern.