Make `FirstEvent` configurable to better support public chain with multi-party

As per the discussion here: https://discord.com/channels/905194001349627914/1306104244486668369/1306929797296816200, Firefly-cli currently hard code the starting block of the Firefly batch pin contract event to `0` to ensure no events could be missed. But this is impractical when using multi-party mode with a public chain: https://github.com/hyperledger/firefly-cli/blob/14e7721bf8cca1fc6b2b217d00a5d79926a17af2/internal/stacks/stack_manager.go#L988

## Workaround

The currently workaround is to update the firefly_core yaml generated manually to start indexing the events from a desired block number. (hint: `newest` can be used if you don't care about missing events which breaks integrity of indexed data)

An enhancement would be making the FirstEvent configurable