[keyserver] Introduce 0x dependency for keyserver profiling

Summary:
GitHub: https://github.com/davidmarkclements/0x

Suggested in official Node docs: https://nodejs.org/en/docs/guides/diagnostics-flamegraph#use-a-pre-packaged-tool

It was pretty seamless to get this working w/ our project, I was quickly able to generate flamegraphs like:

https://blob.sh/8e38f2.png

Test Plan: NA, just adding dependency

Reviewers: ashoat, tomek, rohan, ginsu

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9300