[sovereign] Multiple incoming events processor

[mariusmihaic]

Reasoning behind the pull request

• Refactor incoming events processor to act as a holder of multiple incoming event processors

Proposed changes

• Moved the switch code from incomingEventsProcessor.go into depositEventProc.go & executedBridgeOpEvent.go
• Now the sovereign sdk users can define their own subscribed incoming events (NotifierConfig.SubscribedEvents from sovereignConfig.toml) and implement the corresponding processor, which will be added by calling RegisterEventHandler
  from incomingHeaderProcessor

Testing procedure

Pre-requisites

Based on the Contributing Guidelines the PR author and the reviewers must check the following requirements are met:

• was the PR targeted to the correct branch?
• if this is a larger feature that probably needs more than one PR, is there a feat branch created?
• if this is a feat branch merging, do all satellite projects have a proper tag inside go.mod?

The base branch was changed.