Use logging filter, remove otel.

[yuroitaki]

This addresses #412

• Switched from using config file to indicate log verbosity level to using RUST_LOG env var which is much more flexible
• Removed opentelemetry setup code which is flooding the logs with SpanData struct because it was temporarily setup to export to stdout — we can remove it for now since we're not using any opentelemetry tool yet
• Fixed the config file logging setting to use logging filter directive instead of just verbosity level

[sinui0]

It's not clear to me whether this is already the case, but let's enable logs for all those dependencies by default (tlsn_verifier and tls_mpc).

I think it would be nice if the filter field is an optional which overrides the level field.

[sinui0]

Looks good, thank you!

[yuroitaki]

By the way @sinui0 @heeckhau please don't merge this first even after all approvals have been obtained when I am away the next few days — since there's a format change of the config file, I'd like to change that in our AWS server first and test before CD kicks in which ll fail otherwise