feat: Make archiving optional

[baolsen]

We have a use case where we cannot do remote builds on Terraform enterprise due to corporate restrictions.

In our case, the contents of source_dir is quite complex, some files must be put in specific locations in the archive.
And we are not allowed to use the build commands to achieve this.

The module does currently allow the build process to be skipped.
But it does not allow any way for us to specify a custom archive entirely.

For this customer, we would prefer to create the archive ahead of time and store it in our repo for this customer. Then tell the module to upload our pre-packaged archive.

For our CI, we still need the existing remote build feature.
We can use this to ensure our stored archive is kept up to date by comparing the two.

We therefore need an optional way to upload an existing archive, rather than the module creating one.

While this does move slightly away from the core benefits of this module, I still believe it is a useful option to have.