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

.github/workflows/unit_tests.yaml

@@ -159,13 +159,13 @@ jobs:
           mkdir -p ${KCOV_ROOT}/build
           cd ${KCOV_ROOT}/build && cmake -Wno-dev ${KCOV_ROOT}/kcov-${KCOV_VERSION} && cd -
           make -C ${KCOV_ROOT}/build && sudo  make -C ${KCOV_ROOT}/build install
-    - name: Bootstrap clingo from sources
+    - name: Bootstrap clingo
       if: ${{ matrix.concretizer == 'clingo' }}
       env:
           SPACK_PYTHON: python
       run: |
           . share/spack/setup-env.sh
-          spack external find --not-buildable cmake bison
+          spack bootstrap untrust spack-install
           spack -v solve zlib
     - name: Run unit tests (full suite with coverage)
       if: ${{ needs.changes.outputs.with_coverage == 'true' }}