Implement reading apply configuration from stdin

[r4victor]

Closes #2304

This PR allows dstack apply to read configurations from stdin instead of files using the syntax dstack apply -y -f -. Configuration files can now be parametrized in arbitrary ways on the client side like this:

type: volume
name: my-vol

backend: aws
region: us-east-1
size: $VOL_SIZE

export VOL_SIZE=50
envsubst '$VOL_SIZE' < .dstack/volume.dstack.yml | dstack apply -y -f -

The -y/--yes flag is currently required for -f - to work.

The PR also fixes a few places where the CLI asked for confirmation even with --yes, e.g. when ctrl+c in dstack apply.