Add support for SSL certificate generation

Add support to request SSL certificates from Vault's PKI backend. Certificates can be issued on demand by using a role.

The generated certificate and the issuing CA certificate need to be stored in a Truststore (Keystore) and the private key in a Keystore. Vault should be used as intermediate CA, so the Truststore should contain the Root CA certificate that should be configured along the certificate request properties.

Config parameters for requesting certificates:

• common-name
• alt-names
• ip-sans

Other required parameters:

• Root certificate

Challenges:

• Public key pinning: Preserve the generated certificate/key pair during its validity period. This is to prevent multiple apps running behind a common endpoint address to use different certificates.