[IMPROVEMENT] Add new PKI routes - Read Issuer Certificate

[jon5477]

Is your feature request related to a problem? Please describe.
This is not related to a problem, but I believe it is a request for adding missing API calls to this driver.

In the PKI section of the Vault API, I believe there is missing functionality for reading the CA certificate for the following routes:

Method	Path	Issuer	Format
GET	/pki/cert/ca	default	JSON
GET	/pki/ca	default	DER
GET	/pki/ca/pem	default	PEM
GET	/pki/issuer/:issuer_ref/json	Selected	JSON
GET	/pki/issuer/:issuer_ref/der	Selected	DER
GET	/pki/issuer/:issuer_ref/pem	Selected	PEM

You can refer to the documentation in the section Read Issuer Certificate

Describe the solution you'd like
If these route implementations already exist, please point me in the correct direction.

Describe alternatives you've considered
I know these routes could be self-implemented in my application, but I wish to contribute these implementations to the OSS driver.

Additional context
I've already begun making some changes here, feedback would be welcomed.