Consolidate event filtering options into a single --event-filter flag

[hayao-k]

Currently, the health-exporter provides several flags for filtering AWS Health events, such as --service, --event-status, and --account-id. While these flags offer granular control over event filtering, having multiple flags can make the command-line interface more complex and harder to use, especially when combining multiple filtering criteria.

To simplify the user experience and improve the tool's usability, it would be beneficial to consolidate all event filtering options into a single --event-filter flag. This flag would accept a comma-separated list of key-value pairs, each representing a specific filtering criterion.

For example:

--event-filter "service=RDS,status=upcoming"