feat: default paths for CLI keygen, proving, and verification

[stephenh-axiom-xyz]

Resolves INT-2862.

• Outputs for keygen and prove are written to a default location if no path is provided
• Inputs for prove and verify are read from a default location if no path is provided
• NOTE: Users still need to specify the location of their config TOML and vmexe.

[linear]

INT-2862 CLI uses current directory by default + setup fixes

keygen/prove/execute/verify CLI commands need to specify lots of arguments about App VM proving key/Exe. We should read/write them into the current directory by default

Also, setup should a) check if solc is installed (and fail quickly with an error prompt if so) and b) download kzg files into ./openvm/params/. We should specify the amount of memory needed to run setup in the warning.

[jonathanpwang]

LGTM