inspect: display builder features

[crazy-max]

While working on #1846, I wanted to know nodes capability for some builders. I think we should display the list of features when inspecting a builder.

$ docker --debug buildx inspect docker2010
Name:   docker2010
Driver: docker

Nodes:
Name:      docker2010
Endpoint:  docker2010
Status:    running
Buildkit:  v0.8.2+eeb7b65
Platforms: linux/amd64, linux/arm64, linux/riscv64, linux/ppc64le, linux/s390x, linux/386, linux/arm/v7, linux/arm/v6
Features:
 Cache export:       false
 Docker exporter:    false
 History API:        false
 Multiple platforms: false
 OCI exporter:       false

$ docker --debug buildx inspect docker2010
Name:          default
Driver:        docker
Last Activity: 2023-05-30 12:30:59 +0000 UTC

Nodes:
Name:      default
Endpoint:  default
Status:    running
Buildkit:  v0.11.6
Platforms: linux/amd64, linux/amd64/v2, linux/amd64/v3, linux/arm64, linux/riscv64, linux/ppc64le, linux/s390x, linux/386, linux/mips64le, linux/mips64, linux/arm/v7, linux/arm/v6
Features:
 Cache export:       false
 Docker exporter:    false
 History API:        true
 Multiple platforms: false
 OCI exporter:       false
GC Policy rule#0:
 All:           false
 Filters:       type==source.local,type==exec.cachemount,type==source.git.checkout
 Keep Duration: 172.8µs
 Keep Bytes:    2.764GiB
GC Policy rule#1:
 All:           false
 Keep Duration: 5.184ms
 Keep Bytes:    20GiB
GC Policy rule#2:
 All:        false
 Keep Bytes: 20GiB
GC Policy rule#3:
 All:        true
 Keep Bytes: 20GiB

[jedevc]

LGTM for when we merge the history API detection 🎉

[tonistiigi]

I guess this is fine although some of these were not designed to be user-visible but just the booleans that client code could use when handling flags. No way is this meant to be the same as BuildKit's cap-based feature detection for example.

[crazy-max]

Now displays features only if debug is enabled.