Adding EnsureBot plugin helper. (#10542)

* Adding EnsureBot plugin helper.

* Removing unessisary GetBot call.

* Moving to own file and error handling cleanup.

* Removing patch functionaliy. Plugins should manage their own bot account updates for now.

* Adding tests and cleaning up errors.

* Modify to not shadow err.

* Moving helpers to seperate interface.

* Feedback fixes

plugin/api.go

@@ -457,6 +457,7 @@ type API interface {
 	// KV Store Section
 
 	// KVSet will store a key-value pair, unique per plugin.
+	// Provided helper functions and internal plugin code will use the prefix `mmi_` before keys. Do not use this prefix.
 	KVSet(key string, value []byte) *model.AppError
 
 	// KVCompareAndSet will update a key-value pair,