cli: add shell completion

[crazy-max]

closes #1674
partially solves #1072

Add completion to:

• List builder names for --builder persistent flag and as arg for some cmds
• Folders (context) for build command
• List bake targets

$ docker __completeNoDesc buildx --builder ""
builder
builder2
buildkit0106
buildkit093
remote-builder
default
desktop-linux
docker201016
docker2300
docker2400
:4
Completion ended with directive: ShellCompDirectiveNoFileComp

$ docker __completeNoDesc buildx --builder "do"
docker201016
docker2300
docker2400
:4
Completion ended with directive: ShellCompDirectiveNoFileComp

For bake, can be tested within this repo:

$ docker __completeNoDesc buildx ""
bake
build
create
du
help
imagetools
inspect
ls
prune
rm
stop
use
version
: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mod-outdated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