rzv CLI 0.21.0: larger sandbox sizes and exact exec arguments

rzv 0.21.0 adds two larger sandbox sizes and runs exec arguments exactly as typed.

• rzv sandboxes create --size 8vcpu-32gb|16vcpu-64gbAccepts the 8vcpu-32gb and 16vcpu-64gb sandbox sizes as well as 2vcpu-8gb and 4vcpu-16gb. Which sizes can start now depends on capacity; rzv sandboxes list shows them.
• rzv sandboxes exec <sandbox-id> -- <command>With two or more arguments after --, quotes each one, so the sandbox runs exactly the argument list you typed. One argument still runs as a shell script; put pipes, && or $VARS in a single quoted argument.
• The sandboxes help describes API key access correctly: sandboxes:read and sandboxes:write are key permissions, granted with rzv api-keys create --permissions, or use the all:all scope.
• Downloads are on GitHub at https://github.com/Runzivo/rzv-cli/releases/latest. Release notes are on the same page.