Replace `*Enum` classes with a native Enums

[jeromegamez]

interface ChannelInterface
{
-    public function getMode(): int;
+    public function getMode(): ChannelMode;
}

That's a breaking change - I don't know if this is acceptable for 0.6.x but I thought I could give it a try anyway 🤞🏻

:octocat:

[WyriHaximus]

That's a breaking change - I don't know if this is acceptable for 0.6.x but I thought I could give it a try anyway 🤞🏻

I mean, this is the amount of breaking changes I made merging the sync and async client into once, breaking changes are fine in 0.6.x: https://github.com/jakubkulhan/bunny/pull/147