spack buildcache push: best effort (#45631)

"spack buildcache push" for partially installed environments pushes all it 
can by default, and only dumps errors towards the end.

If --fail-fast is provided, error out before pushing anything if any
of the packages is uninstalled

oci build caches using parallel push now use futures to ensure pushing
goes in best-effort style.

lib/spack/docs/conf.py

@@ -206,6 +206,7 @@ def setup(sphinx):
     ("py:class", "six.moves.urllib.parse.ParseResult"),
     ("py:class", "TextIO"),
     ("py:class", "hashlib._Hash"),
+    ("py:class", "concurrent.futures._base.Executor"),
     # Spack classes that are private and we don't want to expose
     ("py:class", "spack.provider_index._IndexBase"),
     ("py:class", "spack.repo._PrependFileLoader"),