custom uri fetcher doc (#725)

Co-authored-by: Michael Steinbacher <michael.steinbacher@zeiss.com>

doc/cust-fetcher.md

@@ -1 +1,61 @@
+# Custom URIFetcher
+
 The default `URIFetcher` implementation uses JDK connection/socket without handling network exceptions. It works in most of the cases; however, if you want to have a customized implementation, you can do so. One user has his implementation with urirest to handle the timeout. A detailed discussion can be found in this [issue](https://github.com/networknt/json-schema-validator/issues/240)
+
+## Example implementation
+
+The default URIFetcher can be overwritten in order to customize its behaviour in regards of authorization or error handling.
+Therefore the _URIFetcher_ interface must implemented and the method _fetch_ must be overwritten.
+
+```
+public class CustomUriFetcher implements URIFetcher {
+
+    private static final Logger LOGGER = LoggerFactory.getLogger(CustomUriFetcher.class);
+
+    private final String        authorizationToken;
+
+    private final HttpClient    client;
+
+    public CustomUriFetcher(String authorizationToken) {
+        this.authorizationToken = authorizationToken;
+        this.client = HttpClient.newBuilder().connectTimeout(Duration.ofSeconds(10)).build();
+    }
+
+    @Override
+    public InputStream fetch(URI uri) throws IOException {
+        HttpRequest request = HttpRequest.newBuilder().uri(uri).header("Authorization", authorizationToken).build();
+        try {
+            HttpResponse<String> response = this.client.send(request, HttpResponse.BodyHandlers.ofString());
+            if ((200 > response.statusCode()) || (response.statusCode() > 299)) {
+                String errorMessage = String.format("Could not get data from schema endpoint. The following status %d was returned.", response.statusCode());
+                LOGGER.error(errorMessage);
+            }
+
+            return new ByteArrayInputStream(response.body().getBytes(StandardCharsets.UTF_8));
+        } catch (InterruptedException e) {
+            throw new RuntimeException(e);
+        }
+    }
+}
+```
+
+Within the _JsonSchemaFactory_ the custom URIFetcher can be referenced.
+This also works for schema references ($ref) inside the schema.
+
+```
+...
+CustomUriFetcher uriFetcher = new CustomUriFetcher(authorizationToken);
+
+JsonSchemaFactory schemaFactory = JsonSchemaFactory.builder()
+        .uriFetcher(uriFetcher, "http")
+        .addMetaSchema(JsonMetaSchema.getV7())
+        .defaultMetaSchemaURI(JsonMetaSchema.getV7().getUri())
+        .build();
+JsonSchema jsonSchema = schemaFactory.getSchema(schemaUri);
+for (ValidationMessage validationMessage : jsonSchema.validate(jsonNodeRecord)) {
+    // handle the validation messages
+}
+```
+
+**_NOTE:_** 
+Within `.uriFetcher(uriFetcher, "http")` your URI must be mapped to the related protocol like http, ftp, ...