Added HTTP requests rate lmiting

[wxifze]

Sometimes a download fails with an error message in the form:

The Album/Track requested does not exist at: https://...

This happens during metadata fetch, before downloading the actual audio and image files. On the first run in a while, this error appears several minutes into the run. On a consecutive run, a different error appears immediately:

WARNING:bandcamp-dl.Main:Could not find music grid on the page. No albums found.

I reproduced this behavior from two geographically distinct IP addresses when downloading an artist with a large number of tracks using the --embed-* options:

bandcamp-dl --embed-lyrics --embed-art --embed-genres --artist adarook

This happens when the script reaches a per-IP request rate limit and the server begins to return 429 Too Many Requests for some requests. This can be confirmed by running the script with --debug or visiting bandcamp.com from a browser with the same IP.

This PR implements an optional global limit on the number of requests sent per minute.

It introduces a new config option, limit_req_per_minute, and a command line argument, --limit-req-per-minute, with equivalent semantics. The default value 0 disables rate limiting. Positive integers constrain the number of requests sent per minute using a leaky-bucket algorithm.

I did not test this on Python versions other than 3.13.7. This change adds a new dependency, requests-ratelimiter.