[Java][Client] Add support for the new Spring RestClient (#18522)

* feat (JAVA SPRING RESTTEMPLATE) 17571: initial commit for Spring RestClient

* feat (JAVA SPRING RESTTEMPLATE) 17571: Copied and changed the webclient mustache files

The RestClient API is oriented on the WebClient API so many parts of the templates can be the same

* fix (JAVA SPRING RESTTEMPLATE) 17571: Renaming error & add README template

The README must be changed because the minimal Java Version for this client is 17

* fix (JAVA SPRING RESTTEMPLATE) 17571: Imports, compile errors and cookie setting

* feat (JAVA SPRING RESTTEMPLATE) 17571: Add generated samples

* test (JAVA SPRING RESTTEMPLATE) 17571: Add tests

* feat (JAVA SPRING RESTTEMPLATE) 17571: Update doc

* Add the restcilent to samples-java-client-jdk17.yam

The minimum Java version of the used Spring Version is 17

* fix (JAVA SPRING RESTTEMPLATE) 17571: Workflow paths to petstore samples

* fix (JAVA SPRING RESTTEMPLATE) 17571: Regenerated samples

* feat (JAVA SPRING RESTTEMPLATE) 17571: Generated echo-api sample for RestClient

* fix (JAVA SPRING RESTTEMPLATE) 17571: Missing import

* fix (JAVA SPRING RESTTEMPLATE) 17571: Missing body class exception when null body is set

* test (JAVA SPRING RESTTEMPLATE) 17571: Add echo-api Auth test

* fix (JAVA SPRING RESTTEMPLATE) 17571: Gradlew file permissions

* feat (JAVA SPRING RESTTEMPLATE) 17571: Reggenerate samples after rebase

* test (JAVA SPRING RESTTEMPLATE) 17571: Add echo-api Body gif test

* test (JAVA SPRING RESTTEMPLATE) 17571: Add echo-api octet stream body test

* test (JAVA SPRING RESTTEMPLATE) 17571: Add echo-api multipart form data test

* fix (JAVA SPRING RESTTEMPLATE) 17571: Form as body when body is null

Also regenerated the restclient samples

.github/workflows/samples-java-client-echo-api-jdk17.yaml

@@ -0,0 +1,42 @@
+name: Java Client (Echo API) JDK17
+
+on:
+  push:
+    paths:
+      - samples/client/echo_api/java/restclient
+  pull_request:
+    paths:
+      - samples/client/echo_api/java/restclient
+jobs:
+  build:
+    name: Build Java Client JDK17
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        sample:
+          # clients
+          - samples/client/echo_api/java/restclient
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-java@v4
+        with:
+          distribution: 'temurin'
+          java-version: 17
+      - name: Cache maven dependencies
+        uses: actions/cache@v4
+        env:
+          cache-name: maven-repository
+        with:
+          path: |
+            ~/.m2
+          key: ${{ runner.os }}-${{ github.job }}-${{ env.cache-name }}-${{ hashFiles('**/pom.xml') }}
+      - name: Setup node.js
+        uses: actions/setup-node@v4
+      - name: Run echo server
+        run: |
+          git clone https://github.com/wing328/http-echo-server -b openapi-generator-test-server
+          (cd http-echo-server && npm install && npm start &)
+      - name: Build
+        working-directory: ${{ matrix.sample }}
+        run: mvn clean package