[azsdk-cli] Handle versioning

Versioning's a big topic, and there are many facets, but we have some very practical things we need to consider:

1. azsdk-cli depends on things in `eng`, like scripts and probably more as time goes on. This implies that `eng` _and_ the cli needs to be versioned together. @timovv brought up that do this today with test-proxy, so the same policy should apply here.
2. azsdk-cli does depend on some external tools, which we use via `npx`, or other means. We should take a call on whether those dependencies should be versioned. At the very least, we can make sure that the tool versions are stored centrally, so we can manage them together, even if all of them are just pointing to latest for now.

Item 2 might just create an appropriate requirements.txt or package.json to ensure that our tools are locked to the same version each time, and only upgrade when we decide to. We don't have to reinvent the wheel there.

Start by reading the versioning discussion, the eng scripts, and the existing test-proxy versioning policy mentioned in the issue. Review how azsdk-cli currently invokes external tools through npx or other means. Done means agreeing on a versioning policy and documenting or implementing centralized dependency and eng/CLI version management.

Written by the indexing model from the issue text.