feat(serve-static): add onFound option

[yusukebe]

This PR introduces the onFound option for Serve Static Middleware.

If you want to add a custom header to found files, you can write the following:

app.get(
  '/static/*',
  serveStatic({
    // ... 
    onFound: (_path, c) => {
      c.header('Cache-Control', `public, immutable, max-age=31536000`)
    },
  })
)

Related to #3395 honojs/node-server#195

The author should do the following, if applicable

• Add tests
• Run tests
• bun run format:fix && bun run lint:fix to format the code
• Add TSDoc/JSDoc to document the code

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.77%. Comparing base (c2b0de4) to head (0727db2).
Report is 2 commits behind head on next.

Additional details and impacted files

@@           Coverage Diff           @@
##             next    #3396   +/-   ##
=======================================
  Coverage   95.77%   95.77%           
=======================================
  Files         154      154           
  Lines        9271     9272    +1     
  Branches     2725     2743   +18     
=======================================
+ Hits         8879     8880    +1     
  Misses        392      392           

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