Unbundle params from output types

[djc]

This is an alternative direction to #326. From here, I think we can add a nice Issuer builder API.

[est31]

I like this one better than #326, but not sure if it has some negative consequences if the params are not available for Certificate.

[djc]

I like this one better than #326, but not sure if it has some negative consequences if the params are not available for Certificate.

The caller could always bundle up the params and the Certificate if they need both, right?

[djc]

@est31 sorry, I just pushed a bunch more stuff -- will stop here and let you take another look.

[blheatwole]

I set my project to use rcgen = { version = "0.13", features = ["aws_lc_rs"], git = "https://github.com/rustls/rcgen", branch="issuer"} which put me at 9e88850.

After a few updates I was able to successfully sign using a mock Key Vault. I'm working on confirming with the real Azure Key Vault.

[cpu]

I'll give this a review by EOW.