@JsonProperty("signingKeyString")
    public String getSigningKeyString() {
        return signingKeyString;
    }
Signing Key or URL to a JWK(S)
<p>
To validate JWT. Must be Base-64 encoded or you specify a URL to a JWK(S)
plugins/jwt-policy/src/main/java/io/apiman/plugins/jwt/beans/JWTPolicyBean.java