feat: add ability to benchmark multiple sources of the same type

[merklefruit]

It would be useful to benchmark different environments, tiers or regions of Fiber at the same time.
This means adding the ability to have more than 1 fiber source.

Implementation

Since we can already have multiple Fiber endpoints to support the multiplexed client,
we can start by adding a --fiber-only flag that:

1. uses each unique fiberEndpoint to setup a new fiber source (with the same api key for now should be fine)
2. configure a second fiber endpoint as "other source" in TransactionBenchmarker