Adding a User-Agent header to socket connections

[holasoyender]

This is a suggestion that was discussed on the Discord support server. Related to lavalink-devs/lavalink-client#34

This pull request adds the option for clients to set a User-Agent header on socket connections to set extra information about the bot.

Headers example

Authorization: youshallnotpass
User-Id: 170939974227541168
Client-Name: lavalink-client/2.0.0
User-Agent: TestBot/1.0.0

When the User-Agent header is set, every log will have a prefix to identify the client its coming from.

[holasoyender]

As there is no actual way of creating a relation between a websocket connection and a http request, the easiest way of implementing it is to read the user-agent from the requests, asking for client library devs to also include that header on every request.

Related to holasoyender/lavalink-client@8dfaf59