cli: Add ability to switch between anchor CLI versions

[armaniferrante]

This is important for allowing many versions of verifiable builds, because if there's a breaking change in the Anchor.toml, then the CLI won't work. If there's a way to switch between anchor CLI versions, then one can easily produce verifiable builds regardless of the Anchor.toml version being used.

[paul-schaaf]

there's also the option of having a separate installer / version manager do this. anchor-vm avm anchor-install ?

although im not experienced enough here to say whether that would be a better option

[tomlinton]

I played around with this and was using a separate cli called avm. It can't be part of the standard anchor cli because you could switch to an earlier version before it was implemented and not be able to switch back.

[tomlinton]

I got reasonably far with this so you can assign it to me if you haven't started work on it 👍

[armaniferrante]

I got reasonably far with this so you can assign it to me if you haven't started work on it +1

Awesome thanks @tomlinton.