Bootstrap clingo from binaries (#22720)

* Bootstrap clingo from binaries

* Move information on clingo binaries to a JSON file

* Add support to bootstrap on Cray

Bootstrapping on Cray requires, at the moment, to
swap the platform when looking for binaries - due
to #22800.

* Add SHA256 verification for bootstrapped software

Use sha256 verification for binaries necessary to bootstrap
the concretizer and gpg for signature verification

* patchelf: use Spec._old_concretize() to bootstrap

As noted in #24450 we may happen to need the
concretizer when bootstrapping clingo. In that case
only the old concretizer is available.

* Add a schema for bootstrapping methods

Two fields have been added to bootstrap.yaml:
  "sources" which lists the methods available for
       bootstrapping software
  "trusted" which records if a source is trusted or not

A subcommand has been added to "spack bootstrap" to list
the sources currently available.

* Methods used for bootstrapping are configurable from bootstrap:sources

The function that tries to ensure a given Python module
is importable now tries bootstrapping methods in the same
order as they are defined in `bootstrap.yaml`

* Permit to trust/untrust bootstrapping methods

* Add binary tests for MacOS, Ubuntu

* Add documentation

* Add a note on bash

etc/spack/defaults/bootstrap.yaml

@@ -5,3 +5,28 @@ bootstrap:
   # Root directory for bootstrapping work. The software bootstrapped
   # by Spack is installed in a "store" subfolder of this root directory
   root: ~/.spack/bootstrap
+  # Methods that can be used to bootstrap software. Each method may or
+  # may not be able to bootstrap all of the software that Spack needs,
+  # depending on its type.
+  sources:
+  - name: 'github-actions'
+    type: buildcache
+    description: |
+      Buildcache generated from a public workflow using Github Actions.
+      The sha256 checksum of binaries is checked before installation.
+    info:
+      url: https://mirror.spack.io/bootstrap/github-actions/v0.1
+      homepage: https://github.com/alalazo/spack-bootstrap-mirrors
+      releases: https://github.com/alalazo/spack-bootstrap-mirrors/releases
+  # This method is just Spack bootstrapping the software it needs from sources.
+  # It has been added here so that users can selectively disable bootstrapping
+  # from sources by "untrusting" it.
+  - name: spack-install
+    type: install
+    description: |
+      Specs built from sources by Spack. May take a long time.
+  trusted:
+    # By default we trust bootstrapping from sources and from binaries
+    # produced on Github via the workflow
+    github-actions: true
+    spack-install: true