feat: pull all bake targets by default

[goller]

Previously, only a single named bake target would be pulled. This was awkward as one would need to know all targets and then loop.

Now, depot pull will download all targets simultaneously by default.

The names of the targets should correspond to those used by docker compose and therefore one should be able to immediately run docker compose up.

The --target option for depot pull has been changed to accept more than one string and now acts like a filter for the bake images to download.