core(runner) - Add option to use an existing terraform installation

[Minosity-VR]

What does this PR do?

Enhancement: Support STRATUS_TERRAFORM_BINARY_PATH env var to use a pre-installed binary. Falls back to downloading if the existing binary version is below the required minimum.

Tested by using a local terraform version, it doesn't reinstall one.

Motivation

Soon™️ we are going to open source a wrapper for stratus to run it as a service. It requires some modifications in the core:

• control over the runner internals
• remote state for terraform
• context propagation to handle cancellation
• logger control

Having a controlable terraform binary rather than relying on the runner to download one makes it easier to deploy as a service - no request to hashicorp's domain that might need CNP authorisation + upcoming changes to allow a remote state would have conflicted - if using a remote state, it would end up storing the terraform binary remotely

[NathanaeldeLaChesnais]

Do we want to mock tests for existingBinaryVersionSufficient?

[Minosity-VR]

Do we want to mock tests for existingBinaryVersionSufficient?

If we mock, we will just return an arbitrary version instead of calling the real terraform. Then what we will really test is just that version.Must behaves correctly, and that's just an imported package so I think we can rely on hashicorp's test