adding json export for spack blame (#23417)

I would like to be able to export (and save and then load programatically)
spack blame metadata, so this commit adds a spack blame --json argument,
along with developer docs for it

Signed-off-by: vsoch <vsoch@users.noreply.github.com>

Co-authored-by: vsoch <vsoch@users.noreply.github.com>

share/spack/spack-completion.bash

@@ -384,7 +384,7 @@ _spack_arch() {
 _spack_blame() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -t --time -p --percent -g --git"
+        SPACK_COMPREPLY="-h --help -t --time -p --percent -g --git --json"
     else
         _all_packages
     fi