Add command to export credentials to env vars

[antonbabenko]

After login using spacectl profile login <something>, it would be great to be able to export these credentials as env variables expected by Spacelift provider for Terraform - SPACELIFT_API_KEY_ENDPOINT, SPACELIFT_API_KEY_ID and SPACELIFT_API_KEY_SECRET.

This way, it is possible to set credentials automatically depending on the current dir (e.g., I use precmd() already for this) or by running a command explicitly like this:
exec $(spacectl profile export <something>)