Ability to override terraform version per Workspace with environment variable

[ytsarev]

Description of your changes

• Inspired by Tfenv support #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:

• 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.

[jeanduplessis]

At this point in time, we will not allow overriding the Terraform binary with the possibility of one released after 1.5.x as they are released under the BSL license. This provider would violate the license terms, and we do not want to open the possibility for that even if the action was taken by a user of the provider.