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.sum

@@ -0,0 +1,10 @@
+github.com/golang-jwt/jwt/v5 v5.2.2 h1:Rl4B7itRWVtYIHFrSNd7vhTiz9UpLdi6gZhZ3wEeDy8=
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+github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
+github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
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+github.com/google/jsonschema-go v0.2.1-0.20250825175020-748c325cec76/go.mod h1:r5quNTdLOYEz95Ru18zA0ydNbBuYoo9tgaYcxEYhJVE=
+github.com/yosida95/uritemplate/v3 v3.0.2 h1:Ed3Oyj9yrmi9087+NczuL5BwkIc4wvTb5zIM+UJPGz4=
+github.com/yosida95/uritemplate/v3 v3.0.2/go.mod h1:ILOh0sOhIJR3+L/8afwt/kE++YT040gmv5BQTMR2HP4=
+golang.org/x/tools v0.34.0 h1:qIpSLOxeCYGg9TrcJokLBG4KFA6d795g0xkBkiESGlo=
+golang.org/x/tools v0.34.0/go.mod h1:pAP9OwEaY1CAW3HOmg3hLZC5Z0CCmzjAF2UQMSqNARg=