Add Dev Container support (#337)

Fix #324 

This comes with Java 21, which is not fully supported (see #335 ), but
that should eventually be solved anyway.

This will allow contributors to select "open in CodeSpaces" and work
more easily on the project, like here:

![Screenshot 2023-12-08 at 13 11
43](https://github.com/langchain4j/langchain4j/assets/316835/ecf67ec8-69f3-4b85-8e53-4d71773e6768)
![Screenshot 2023-12-08 at 13 12
35](https://github.com/langchain4j/langchain4j/assets/316835/db0fe7c8-1d1a-4ce9-96e8-7f0a5eeface1)

.devcontainer/devcontainer.json

@@ -0,0 +1,25 @@
+{
+  "name": "Java",
+  "image": "mcr.microsoft.com/devcontainers/base:ubuntu",
+  "features": {
+    "ghcr.io/devcontainers/features/java:1": {
+      "version": "21-oracle",
+      "jdkDistro": "oracle"
+    },
+    "ghcr.io/devcontainers/features/azure-cli:1": {},
+    "ghcr.io/devcontainers/features/docker-in-docker:2": {},
+    "ghcr.io/devcontainers/features/github-cli:1": {}
+  },
+
+  "customizations": {
+    "vscode": {
+      "settings": {},
+      "extensions": [
+        "redhat.vscode-xml",
+        "visualstudioexptteam.vscodeintellicode",
+        "vscjava.vscode-java-pack"
+      ]
+    }
+  },
+  "remoteUser": "vscode"
+}