refactor: improve x/rewards codebase and tests

[RiccardoM]

Description

This PR improves a bit the codebase of x/rewards by adding missing comments as well as adding tests and improving existing ones.

---

Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

Reviewers Checklist

All items are required. Please add a note if the item is not applicable and please add
your handle next to the items reviewed if you only reviewed selected items.

I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[hallazzang]

@RiccardoM Why did you move this event emission to here?

[RiccardoM]

It's because this event should only be emitted when a message to create a plan is created, like all other message handlers. We should emit events inside the Keeper only if one method is called by multiple parties and we are interested in that event because of different reasons. This was not the case, and thus I have moved it inside the msg_server to have the same structure as other message handlers.

Also, by emitting events inside the msg_server rather than the keeper, it's easier to quickly identify whether some events are missing or not. This was actually the case for another handler that was not emitting any event.