Support for Elliptic Curve certificate

[scabana]

Hello!

We have a requirement to support ECDSA certificates, I tried adding

policy.KeyType = CertificateKeyType.Ec;
policy.KeyCurveName = CertificateKeyCurveName.P256;
policy.KeySize = 256;

here and got an EC based certificate.

I'd be willing to open a PR to add two parameters to the order command that would look like:

--elliptic-curve P256|P384|P521|P256K
--key-size int

At the same time, I'd add a delay somewhere after

az-acme-cli/src/AzAcme.Cli/Commands/OrderCommand.cs

Line 71 in 6b9f912

await dnsZone.SetTxtRecords(order);

I've been having failures with what looks like dns propagation issues. Same as #28 and #18.

Thanks in advance!

[ross-mcdermott]

Thanks @scabana a PR would be much appreciated!