[Stream] Parameters for stream template runs

Currently, each time a benchmark / stream command is run, a template is referenced and it is used. It happens from time to time, that I want to tweak just 1-2 values be it the total number of events or the values of an enum. Currently there is no option to pass in params on the CLI.

Being able to pass in values on the CLI would also have the effect, that `elastic-package benchmark stream` could be used to simluate many shippers. As an example, I could run elastic-package 3 times and each time I pass in a sligthly different host name for the Elastic Agent and the number of events that would be created. Normally I would need to create 3 templates to make this happen.

Start at the implementation of the `elastic-package benchmark stream` command and trace how it loads and applies templates. Determine how command-line parameters should override template values, then verify that repeated runs can use different host names, enum values, and event counts without requiring separate templates.

Written by the indexing model from the issue text.