Add the ability to customise the details of the CA

[lhaig]

Allow operators to customise the CA created using nomad tls ca create

[jrasell]

I have added some initial comments inline along with an internal note about a change I believe is required for this to work. It would also be nice if new tests used must rather than require, so we continue moving towards this library.

[jrasell]

Thanks for the changes @lhaig; I left some comments inline, mostly stylistic ones and some areas we could reduce the amount of code.

I feel we could make some changes to the validation logic as well after testing this locally. I would expect to be able to modify only the organizational-unit for example, however, when running this I get an error:

./pkg/darwin_arm64/nomad tls ca create --organizational-unit="nomad engineering"
please provide the -common-name flag when customizing the CA

I therefore think we need some type of merge function, which merges default values onto a potentially partially populated opts struct.

[jrasell]

Some inline comments. It would probably also be nice to expand on the command description, given that defaults don't take effect if you modify certain parameters.

[jrasell]

LGTM, thanks so much @lhaig!