fix(gateway): use resume_gateway_url when reconnecting

[NickIsADev]

This pull requests implements the missing handling of resume_gateway_url, which was added to the Gateway in August of 2022 (https://docs.discord.com/developers/change-log#session-specific-gateway-resume-urls). The system was pushed into full use later in September of the same year.

This system does not exist for the Voice Gateway at the moment, hence why no changes were made there.

Changes:

• Store Shard._resume_gateway_url when receiving it via the READY event
• Use Shard._resume_gateway_url to reconnect if available, otherwise use original gateway URL

Documentation Reference: https://docs.discord.com/developers/events/gateway#preparing-to-resume

[NickIsADev]

I've also noticed that Discordia does not properly produce jitter on the initial heartbeat, however I'm unsure of how to approach this issue while keeping the code clean and readable. I've thought of just using sleep(heartbeat_interval * math.random) or setTimeout, however that could lead to even more issues with many heartbeat timers running if multiple reconnects happen quickly (for whatever reason).

Documentation Reference: https://docs.discord.com/developers/events/gateway#heartbeat-interval