Ability to override terraform version per Workspace with environment variable

[ytsarev]

Description of your changes

• Inspired by https://github.com/upbound/provider-terraform/pull/82 CC @bdwyertech

• Add https://github.com/tofuutils/tenv/ to the provider image, a Golang-based successor to https://github.com/tfutils/tfenv

• Manage the terraform cli version with it, default to the current 'safe' 1.5.5

• Document usage

• Overriding the custom version of terraform per Workspace is as easy as

  env:
    - name: TFENV_TERRAFORM_VERSION
      value: 1.4.0 #custom version per workspace

  the tenv will handle the installation and invocation of the custom version

• Within provider pod the multiple version are stored in /.tenv and looks like this in case of multiple version usage

  ~ $ cd .tenv/
  /.tenv $ tree
  .
  └── Terraform
  ├── 1.4.0
  │   └── terraform
  ├── 1.5.5
  │   └── terraform

• /.tenv/Terraform/1.5.5/ is the current default version and prepackaged to the provider image

Fixes #84

I have:

• [x] Run make reviewable to ensure this PR is ready for review.

How has this code been tested

Running custom provider image with this change and invoking several workspace examples

k get workspaces.tf.upbound.io
NAME                           SYNCED   READY   AGE
sample-inline-with-env         True     True    26m
sample-inline-own-tf-version   True     True    19m

Checked the associated contents of /.tenv directory and custom terraform cli invocation in the process list - it worked as expected.

[bdwyertech]

Hey this looks really cool, thanks for the shout out @ytsarev

[ytsarev]

/test-examples="examples/workspace-inline-aws.yaml"

[turkenf]

/test-examples="examples/workspace-inline-aws.yaml"

FYI: This test comment was just left to make sure pipelines work.