Add support for pem certificates

[timja]

Azure Key Vault supports two formats for certificates, PEM and PFX (aka PKCS#12).

This adds support for PEM

Fixes https://github.com/jenkinsci/azure-keyvault-plugin/issues/231

Testing done

Created a certificate in key vault with PEM content type Loaded it in a pipeline with:

node {
     theSecrets = [

                [ secretType: 'Secret', name: 'aaaa', envVariable: 'slackToken'],
                [envVariable: 'CODESIGNCERT', name: 'my-cert', secretType: 'Certificate'],
                [envVariable: 'CODESIGNCERT2', name: 'pem-cert', secretType: 'Certificate']
            ]
           // node {
            azureKeyVault(theSecrets) {
                echo env.slackToken
                echo env.CODESIGNCERT.substring(0, env.CODESIGNCERT.size() - 1)

                //sh "cat $CODESIGNCERT2"

                echo env.CODESIGNCERT2.substring(0, env.CODESIGNCERT2.size() - 1)
            } 
}

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