adding support for export of private gpg key (#22557)

This PR allows users to `--export`, `--export-secret`, or both to  export GPG keys
from Spack. The docs are updated that include a warning that this usually does not
need to be done.

This addresses an issue brought up in slack, and also represented in #14721.

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

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

share/spack/spack-completion.bash

@@ -1010,7 +1010,7 @@ _spack_gpg_sign() {
 _spack_gpg_create() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help --comment --expires --export"
+        SPACK_COMPREPLY="-h --help --comment --expires --export --export-secret"
     else
         SPACK_COMPREPLY=""
     fi
@@ -1027,7 +1027,7 @@ _spack_gpg_init() {
 _spack_gpg_export() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help"
+        SPACK_COMPREPLY="-h --help --secret"
     else
         _keys
     fi