feat(Rest): custom bucket support

[didinele]

Please describe the changes this PR makes and why it should be merged: This PR makes buckets an abstract class that can be implemented. You can now write your own BaseBucket implementation, or use the built-in ProxyBucket for easily sending requests to a proxy server while still using this package.

Status and versioning classification:

• Code changes have been tested against the Discord API, or there are no code changes
• This PR changes the library's interface (methods or parameters added)
• This PR includes breaking changes (methods removed or renamed, parameters moved or removed)