feat: emit helper packet events for indexing pending packets

[Farhad-Shabani]

Closes: #63

Description

• To derive the string representation of the key bytes, I opted for base64 encoding due to its perceived safety, and a more human-readable output.
• The add_event method for publishing IBC events has been superseded by the newly exposed emit_event method.

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PR author checklist

• Linked to GitHub issue.
• Added tests.
• Updated code comments and documentation (e.g., docs/).
• Tagged one reviewer who will be the one responsible for shepherding this PR.

[Farhad-Shabani]

Merging this PR so we can have the typed events along with auxiliaries in our next sov-rollup-starter update. In case the logic of constructing aux events should be improved, we will consolidate it later.