Add eb events --since and --tail

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### Tell us about your request

Please add `--since=TIMESTAMP` and/or `--tail=LINES` options to `eb events` command.

I would like to follow logs since some timestamp (for example deploy started), similar to kubectl has.

I would also accept `--tail=0` option to start following for new entries only.

### Additional context

I'm attempting to build CI workflow that starts update an environment and print events as they occur until deploy is finished.

Start with the implementation of the `eb events` command and trace how it retrieves and follows environment events. Add support for `--since=TIMESTAMP` and `--tail=LINES`, including `--tail=0` for new entries only, and verify that the command can follow events for CI workflows until deployment finishes.

Written by the indexing model from the issue text.