added: Differentiate login methods and more (#1471)

- Added Portuguese in the table (README.md)
- ApplicationProperties.class diluted, provider outsourced to its own class
- Added UnsupportedProviderException to indicate a meaningful error
- Closes #1357
- Closes #1238

src/main/java/stirling/software/SPDF/model/provider/KeycloakProvider.java

@@ -0,0 +1,106 @@
+package stirling.software.SPDF.model.provider;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.stream.Collectors;
+
+import stirling.software.SPDF.model.Provider;
+
+public class KeycloakProvider extends Provider {
+
+    private String issuer;
+    private String clientId;
+    private String clientSecret;
+    private Collection<String> scopes = new ArrayList<>();
+    private String useAsUsername = "email";
+
+    @Override
+    public String getIssuer() {
+        return this.issuer;
+    }
+
+    @Override
+    public void setIssuer(String issuer) {
+        this.issuer = issuer;
+    }
+
+    @Override
+    public String getClientId() {
+        return this.clientId;
+    }
+
+    @Override
+    public void setClientId(String clientId) {
+        this.clientId = clientId;
+    }
+
+    @Override
+    public String getClientSecret() {
+        return this.clientSecret;
+    }
+
+    @Override
+    public void setClientSecret(String clientSecret) {
+        this.clientSecret = clientSecret;
+    }
+
+    @Override
+    public Collection<String> getScopes() {
+        if (scopes == null || scopes.isEmpty()) {
+            scopes = new ArrayList<>();
+            scopes.add("profile");
+            scopes.add("email");
+        }
+        return scopes;
+    }
+
+    @Override
+    public void setScopes(String scopes) {
+        this.scopes =
+                Arrays.stream(scopes.split(",")).map(String::trim).collect(Collectors.toList());
+    }
+
+    @Override
+    public String getUseAsUsername() {
+        return this.useAsUsername;
+    }
+
+    @Override
+    public void setUseAsUsername(String useAsUsername) {
+        this.useAsUsername = useAsUsername;
+    }
+
+    @Override
+    public String toString() {
+        return "Keycloak [issuer="
+                + issuer
+                + ", clientId="
+                + clientId
+                + ", clientSecret="
+                + (clientSecret != null && !clientSecret.isEmpty() ? "MASKED" : "NULL")
+                + ", scopes="
+                + scopes
+                + ", useAsUsername="
+                + useAsUsername
+                + "]";
+    }
+
+    @Override
+    public String getName() {
+        return "keycloak";
+    }
+
+    @Override
+    public String getClientName() {
+        return "Keycloak";
+    }
+
+    public boolean isSettingsValid() {
+        return isValid(this.getIssuer(), "issuer")
+                && isValid(this.getClientId(), "clientId")
+                && isValid(this.getClientSecret(), "clientSecret")
+                && isValid(this.getScopes(), "scopes")
+                && isValid(this.getUseAsUsername(), "useAsUsername");
+    }
+}