Added SendFormAsync in order to send url encoded content. It uses the bucket system all the oder SendAsync methods use.

Added a GetTokenAsync method. This method  is used to get either a token from a code, or a token from a refresh token. It raises an error if given any other kind of tokens.

This means that it moves the useProxy boolean parameter here, not sure how much that affects the rest of the library. def an addition to consider. And something that more accustomed users/contributors will be able to answer.

The main advantage of having such a method in the future could be adding resiliency. Major resiliency by for example using Polly in a simple way see: https://docs.microsoft.com/en-us/dotnet/architecture/microservices/implement-resilient-applications/implement-http-call-retries-exponential-backoff-polly for example.

Of course this means that the user would have to use DI (which is anyways a good practice), but I left the other default ones so that it wouldn't bring any breaking changes.

* Throw exception with CustomStatus

* Add remark instead of exception

* Change wording

Fix breaking change introduced by #1521

* fixed a deadlock in DiscordShardedClient during a failed Identify due to InvalidSession

* fixed log

* Don't wait ready before releasing semaphore

Co-authored-by: Paulo <pnmanjos@hotmail.com>

* Implemented Pending property

* Implemented changes

* feature: Add MessageFlags and AllowedMentions to Modify

* Change exception message