Allow specifying a 'primary' domain for the certificate

[casselc]

Currently the ordering of domains supplied to New-ACMEOrder is not preserved, because the AcmeOrder object is created from the server response which, in practice, appears to return the Identifiers sorted alphabetically.

These updates:

• Add a PrimaryDomain field to the AcmeOrder class
• Updates AcmeOrder constructor logic to initialize PrimaryDomain to the first Identifier so it should be compatible with old CliXML exports.
• Add a PrimaryDomain parameter to the New-Order cmdlet
• Updates the logic of the New-Order cmdlet to set the initial value of PrimaryDomain on the AcmeOrder to the value of the PrimaryDomain parameter if specified, or the value of the first supplied Identifier otherwise.
• Add a primaryDomain parameter to the GenerateCsr method that is used as the CN/Subject of the certificate

[glatzert]

I think there should also be a new Cmdlet Set-Order, which allows setting the PrimaryDomain.
This is not essential, since getting the order from State, changing it and Store it in the state can be done manually, but I think it would be convenient.
Also the PrimaryDomain might be a good candidate to implement a getter and a setter, since it is (probably) crucial, that it's part of the SAN list.

[casselc]

Re: Set-Order and getter/setter - I can do that, but it will only be effective if run prior to Complete-Order since the server Order is immutable after creation, which leads to...

I was wondering if it might not be better to leave AcmeOrder matching the protocol object and instead add something like a new CertificateRequest class and associated cmdlets that would encapsulate the primary domain, SAN list, key settings, any other CSR/export options (Friendly Name,etc) and the active AcmeOrder associated with the request.

[glatzert]

I've seen you implemented the PrimaryName on Complete-Order at first - this would also make any change of the Order object unneccesary. Also there the option to merge (instead of reset) the list of identifiers on Update-Order, so the first identifier is always the same.

Nevertheless the CertificateRequest Object might be an Option, but I'd like to keep the order as the "principal object". So an order could have a CSRSettings. I did not dig too deep into the Certificate Issuance of LE or what is supported in CSR and what not, so if it's more than just the CN, that could be an interesting approach. FriendlyName is - as far as I know - more a Windows thing, is it?
That could be added, but we'd have to be careful about availabiltiy on other Platforms.

I'll open an Issue to discuss this further.

[glatzert]

Closing this, since #18 and #19 implement this by allowing to set a CertDN