feat: add stamp support for pkg_rpm Release tag (#1039)

* feat: add stamp support for pkg_rpm Release tag

Allow `{VARIABLE_NAME}` placeholders in the `release` attribute of
`pkg_rpm`. When `stamp = 1` (or `stamp = -1` with `--stamp`), workspace
status variables from volatile-status.txt are substituted at build time.

- Add `get_status_vars()` to `pkg/private/build_info.py`
- Add `--volatile_status_file` and `--stable_status_file` args to
  `make_rpm.py`; apply substitution in `RpmBuilder.__init__` via the
  new `stamp_vars` parameter
- Add `stamp` and `private_stamp_detect` attrs to the `pkg_rpm` rule;
  rename the rule to `_pkg_rpm_rule` and expose a `pkg_rpm` macro that
  injects `private_stamp_detect` via `select()`, matching the pattern
  used by `pkg_tar` and `pkg_zip`
- Avoid embedding template placeholders in the output filename when stamp
  is active

Fixes #1038

* clean up uses of str.strip() in build_info.py

We don't need to call it multiple times on the same line value, we can
just call it once and save it.

* Address PR #1039 review feedback

- Add docstring to get_status_vars() in build_info.py
- Use get_stamp_detect() from util.bzl instead of inline select()
  to match the pattern from PR #1035
- Expand release attr docstring to document stamp variable substitution
- Add stamped RPM example to examples/rpm/system_rpmbuild_pfg