Make RosettaClientParams.baseUrl required

[rhyslbw]

I'll be sure to check this library out in more depth, but my cursory review surfaced a minor but important issue regarding adoption. The fallback to a specific server might be a nice convenience for Lunar clients, but it's a boundary violation, and would be confusing for other implementations. I'd suggest just requiring the value passed in explicitly, and would be happy to open a PR if you agree.

Thanks for the work so far!

[stuartwk]

Hey @rhyslbw thanks for the feedback! I agree, it's a bit less confusing if we simply make the baseUrl required. If you feel like doing a PR, we'd gladly merge it in.