On platforms with rebind support but no support or TD opts out for
SERVTD_EXT, TDG.servtd.rd on the SERVTD_EXT fields in TDCS of a target
TD would return zeros. This change reads TDCS.ATTRIBUTES to check bit 17
and makes SERVTD_EXT optional throughout the rebinding flow:

- read_servtd_ext() reads TDCS.ATTRIBUTES via tdcall_servtd_rd and
  returns None when SERVTDEXT bit is not set
- write_approved_servtd_ext_hash() accepts Option and is a no-op
  when None
- Certificate generation/verification: servtd_ext extension is
  conditionally included and tolerated when missing
- Policy verification (authenticate_rebinding_old) skips init report
  verification entirely when servtd_ext is unavailable, since the
  init TD report cannot be verified without servtd_info_hash
- SPDM VDM messages send zero-length servtd_ext element when not
  available; receiver handles it gracefully
- All TDCS write operations (write_servtd_rebind_attr,
  write_approved_servtd_ext_hash) are skipped when servtd_ext is None

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Signed-off-by: Haitao Huang <haitaohuang@microsoft.com>