Add stamp attribute to pkg_tar (in the style of cc_binary(stamp=-1,0,1)

[aiuto]

Part 1 of #287 This is not pretty, because Bazel does not make --stamp available directly

• add a config setting to mimic --stamp: private_stamp_detect
  • This is a bit of a hack, until the value of stamp is available directly from starlark ctx
  • Allow Starlark rules to observe the --stamp setting bazel#11164
  • use the config setting to pass a bool down to the packaging rule to see that --stamp was set
• add the stamp attribute to pkg_tar
• use combination of stamp & private_stamp_detect to invoke stamping
• this uses an undocumented feature to get to bazel-out/volatile-status.txt
  • that is bad in theory
  • In practice Bazel will eventually have admit the cat is out of the bag and document the files.

Future work:

• implement for all 4 package types.
• contain a test where a stamped package is used from a workspace that includes rules_pkg.

RELNOTES: Add stamp to pkg_tar.

[aiuto]

@nacl This is ready now. It's not as ugly as I thought it would be.

[nacl]

LGTM -- not particularly ugly, other than the private_stamp_detect attribute.

Most comments other than about private_stamp_detect are mostly nits and documentation suggestions.

Also needs a rebase/merge against main.