[sovereign] Fix shard id computations

[mariusmihaic]

Reasoning behind the pull request

• Fix shard ID computation for sovereign chain
  ProcessDestinationShardAsObserver & AssignShardForPubKeyWhenNotSpecified have not been changed, but should work as expected to return SovereignChainShardID as long as config is also correct

Proposed changes

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?