bake: add completion to list targets

[crazy-max]

partially solves #1072
needs docker/cli#4089

Add completion to list bake targets and also folders (context) for build command.

Can be tested within this repo:

$ docker __completeNoDesc buildx ""
bake    Build from a file
build   Start a build
create  Create a new builder instance
du      Disk usage
imagetools      Commands to work on images in registry
inspect Inspect current builder instance
ls      List builder instances
prune   Remove build cache
rm      Remove a builder instance
stop    Stop builder instance
use     Set the current builder instance
version Show buildx version information
:4
Completion ended with directive: ShellCompDirectiveNoFileComp

$ docker __completeNoDesc buildx bake ""
default
validate
meta-helper
_common
lint
validate-vendor
validate-docs
validate-authors
validate-generated-files
update-vendor
update-docs
update-authors
update-generated-files
mod-outdated
test
binaries
binaries-cross
release
image
image-cross
image-local
:4
Completion ended with directive: ShellCompDirectiveNoFileComp

$ docker __completeNoDesc buildx bake "v"
validate
validate-vendor
validate-docs
validate-authors
validate-generated-files
:4
Completion ended with directive: ShellCompDirectiveNoFileComp