Add go.work and and a Terraform toolchain for Bazel

[SirGitsalot]

This is PR 1/n (where n is like 5 or 6) to get to the point where acceptance tests can be run with a single command that doesn't have external dependencies, including checking out and building into the provider repo(s) or installing Terraform. The end goal is:

git clone git@github.com:YourGitHubId/magic-modules.git
cd magic-modules
bazel test //mmv1/products/alloydb:acc_tests

The main changes here are checking in go.work and go.work.sum and adding a toolchain that downloads and makes available a Terraform binary based on the platform you're running on and the Terraform version you want to test against.

This shouldn't have any noticeable effect on either the traditional build or the Bazel build. If it does it indicates a bug in this PR and I'll fix it.

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit 7b7f3db:

Diff report

Your PR hasn't generated any diffs, but I'll let you know if a future commit does.

[SirGitsalot]

It turned out there was a problem: having go.work at the root broke all of the go binaries we have under tools/ and .ci/ because go looks for a go.work file in parent directories when it compiles from a go.mod file, and if the module being compiled isn't in the parent go.work the build fails.

It turns out that go_deps.from_file() will accept a file from a subdirectory, so putting it under build_defs/go.work makes both Bazel and the regular go build happy.

[melinath]

holding off on reviewing for now pending discussion.