examples/server: add example MCP server with JWT and API key (#339)

This commit introduces a new example demonstrating the integration of
authentication middleware with an MCP server. The server supports both
JWT token and API key authentication, along with scope-based access
control for various MCP tools. Key features include token generation
endpoints, in-memory API key storage, and a health check endpoint.

New files added:
- `main.go`: Implements the MCP server and authentication logic.
- `go.mod` and `go.sum`: Manage dependencies for the project.
- `README.md`: Provides setup instructions, available endpoints, and
example usage.

This example serves as a reference for implementing secure access to MCP
tools.

Fixes #330

examples/server/auth-middleware/go.mod

@@ -0,0 +1,15 @@
+module auth-middleware-example
+
+go 1.23.0
+
+require (
+	github.com/golang-jwt/jwt/v5 v5.2.2
+	github.com/modelcontextprotocol/go-sdk v0.3.0
+)
+
+require (
+	github.com/google/jsonschema-go v0.2.1-0.20250825175020-748c325cec76 // indirect
+	github.com/yosida95/uritemplate/v3 v3.0.2 // indirect
+)
+
+replace github.com/modelcontextprotocol/go-sdk => ../../../