Create a contrib folder for user submitted and maintained guidellm output visualization

[ashishkamra]

Is your feature request related to a problem? Please describe.

1. It is difficult to standardize on a specific visualization for guidellm collected metrics as users have different preferences.
2. At the same time, many users may not want to write their if what they are looking is already available in the repo

Describe the solution you'd like
Guidellm ships with its official HTML visualization. Create a contrib section in the repo to accept user submitted visualizations with appropriate CI tests.

Describe alternatives you've considered
Alternative is for users to maintain the visualizations in their own repos but they might be broken as guidellm evolves and it is hard to share

Additional context
Example user created visualization

concurrency-sweep-test.html

[markurtz]

Adding this into the 0.5 so we can scope out what a more extensive solution would look like both for examples as well as contributed pathways for the new refactor plugins