feat: add support for PKI module to generate certificates

[stklcode]

Issue certificates, e.g. using

PkiResponse res = connector.pki().generateCertificateAndKey(
    "example-com",
    PkiRequest.builder()
        .withCommonName("test.example.com")
        .withAltNames("test2.example.com")
        .withIpSans("192.0.2.1")
        .withKeyFormat(PkiRequest.KeyFormat.PKCS8)
        .build()
);

String certPEM = pkiResponse.data().certificate();  // "-----BEGIN CERTIFICATE-----..."
String keyPEM  = pkiResponse.data().privateKey();   // "-----BEGIN PRIVATE KEY-----..."

Various features and fields were added in different versions of Vault, so we re-introduced version guards in the integration test cases.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
80.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud