Fix request header fields too large

[lifez]

I cannot edit on the previous PR so that I copy and open the new PR #637

Summary

This PR adds option to setup custom parameter GATUS_API_READ_BUFFER_SIZE. This param tweaks fiber ReadBufferSize that control max header size (read more here https://github.com/gofiber/fiber/blob/master/docs/api/fiber.md#config).

The problem occurred in my company cause we use gatus on the same domain as app and this causes sending large cookies.

Issue: #636

Checklist

• [x] Tested and/or added tests to validate that the changes work as intended, if applicable.
• [x] Updated documentation in README.md, if applicable.