Build tags for optional features?

### What is the problem you're trying to solve

Optional / experimental features (IPFS, stargz, nydus, overlaybd, etc) do introduce a fair amount of code and a number of tests that implementors may not be interested in (wether with straight nerdctl builds, or custom cli based on the codebase).

This is especially impactful as IPFS/stargz testing for example is very flimsy and failing builds a lot.

Suggestion here would be to introduce build tags for these to make it either to not compile support in / not test it.

WDYT?

Tagging @AkihiroSuda

### Describe the solution you'd like

na

### Additional context

_No response_