Pending Member join notification

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Three parts to the feature:

1. use a discord py background task firing every ~18 hrs https://discordpy.readthedocs.io/en/latest/ext/tasks/

2. which then uses an async request to hit the bungie endpoint https://bungie-net.github.io/#GroupV2.GetPendingMemberships

3. if returned list is not empty send a message to vanguard-warmind channel.

(There should be patterns already for the async bungie request, there is a nice bungie request conv func, and the parsing should be straightforward. The bg async task does not exist yet, but the link above to the API docs has examples)

[RecursiveHook]

Example of using the Bungie request handler: https://github.com/Ghost-Proxy/Voluspa/blob/develop/cogs/members.py#L149-L157

[RecursiveHook]

And then let's create a new cog called roster_reminder.py and go with the below pattern for now. :+1:

from discord.ext import tasks, commands

class RosterReminder(commands.Cog):
    def __init__(self):
        self.index = 0
        self.printer.start()

    def cog_unload(self):
        self.printer.cancel()

    @tasks.loop(hours=18.0)
    async def printer(self):
        print(self.index)
        self.index += 1