feat: configuration engine in workspace

[pchalamet]

Add support for following attributes in workspace block:

• engine: default container tool to use
• configuration: default configuration to use
• environment: default environment to use

Still those values can be overridden on command line.
NOTE: breaking changes for --container-tool renamed as --engine.

The variable terrabuild.engine is now available with following enum values:

• ~docker
• ~podman
• ~none

example:

workspace {
  engine = ~docker
  configuration = "local"
  environment = "dev"
}