feat: Add headers, panic, echo and store utilities

[guikcd]

Closes: #596

• GET /headers Print the heads of the inbound request
• GET /panic Shutdown the application with an error code
• POST /echo Write back the payload sent to it
• POST /store Write the payload to a file and return a hash
• GET /store/{hash} Return the payload from the file system previously stored

My tests:

$ curl localhost:8080/utility/headers
{"Host":["localhost:8080"],"User-Agent":["curl/8.7.1"],"Accept":["*/*"],"X-Session-ID":["ac60f659-0b4e-4d49-9b2a-5f6fbcf8c250"]}

$ curl --include localhost:8080/utility/panic
HTTP/1.1 500 Internal Server Error
Content-Type: text/plain;charset=UTF-8
Content-Length: 16
set-cookie: SESSIONID=ff033cf5-08f6-4f6c-b4c9-4fb89f9b9fdd

Shutting down...

$ curl localhost:8080/utility/echo -X POST -H "Content-Type: application/json" -d '{"store": "store2"}'
{"store": "store2"}

$ curl localhost:8080/utility/store -X POST -H "Content-Type: application/json" -d '{"store": "store1"}'
{"hash": "1698894751"}

$ curl localhost:8080/utility/store/1698894751
{"store": "store1"}

# fill /tmp....
curl localhost:8080/utility/store -X POST -H "Content-Type: application/json" -d '{"store": "store2}"'
HTTP/1.1 500 Internal Server Error
Content-Type: text/plain;charset=UTF-8
Content-Length: 43
set-cookie: SESSIONID=52ab08e3-b084-432a-97c4-7f444eaebcc9

Error writing file: No space left on device

# try reading nonexistent hash
$ curl --include localhost:8080/utility/store/123456789
HTTP/1.1 404 Not Found
Content-Type: text/plain;charset=UTF-8
Content-Length: 67
set-cookie: SESSIONID=6b6855f1-56e2-4818-93bb-16ef5fdbc4ca

Error reading file: /tmp/123456789.json (No such file or directory)
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