bootstrap: add debugging aid to identify the *resolved* config value

[jieyouxu]

I wish we had added a command to bootstrap for debugging the resolved value of each config, not just the ones that have been explicitly overridden.

Originally posted by @jyn514 in #136701

[jyn514]

maybe we could have something like this as the interface:

$ x config
change-id = 133068
build.extended = true
build.submodules = false
rust.debug = true
# ... *all* of the remaining configs
$ x config build
build.extended = true
build.submodules = false
# ... rest of the configs for the build table only
$ x config build.extended
true