added automatic retry handlers for 429 and 5xx error codes, as well as custom exceptions for ratelimiting, client errors, and server errors
also added a more advanced user agent string constructor. previously, setting FloweryAPIConfig.user_agent would override the default user_agent. so, if you set user_agent to `foobar`, the user agent string would be `foobar` in requests. now, the user agent string sent by requests would be the following on my development machine:
`'User-Agent': 'PyFlowery/1.0.3 PyFloweryTests (Python 3.12.6 (main, Sep 8 2024, 13:18:56) [GCC 14.2.1 20240805])'`
