feat(compute): add install-viceroy command to pre-install Viceroy

[mschfh]

Change summary

The container images (Dockerfile-go, Dockerfile-node, Dockerfile-rust) re-download Viceroy on each container start (raised in #1828 (review)), this PR adds an explicit fastly compute install-viceroy command that downloads and
installs the Viceroy local-testing binary.

Implementation

Rather than duplicate the download logic, the existing installer was extracted out into a shared viceroyInstaller (pkg/commands/compute/viceroy.go).

Both compute serve and the new command call it, so every version/override
mechanism (the manifest's [local_server] viceroy_version, FASTLY_VICEROY_USE_PATH)
applies identically during a pre-install. ServeCommand.GetViceroy is now a thin
wrapper, so behavior is unchanged.

This is intentionally simpler than #1805 (which embeds binaries into the CLI binary).

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

User Impact

A new fastly compute install-viceroy command lets users (and the container
images) pre-install Viceroy ahead of time, so compute serve can start without
needing to download it. No change to existing behavior - compute serve still
installs Viceroy on demand if it isn't already present.

Are there any considerations that need to be addressed for release?

None. No breaking changes - the command is purely additive and the shared
extraction keeps compute serve's install path identical.