add dynamic AUTOSAR PDU-container extraction to CAN bus logging

[kipp-ing]

What changed

extract_pdus() added to src/asammdf/blocks/bus_logging_utils.py. It decodes
AUTOSAR dynamic PDU-container CAN frames. A container frame carries a variable
sequence of contained PDUs, each prefixed by a Header_ID and Header_DLC
field. Each contained PDU's byte offset depends on the lengths of the PDUs
before it, so the payload is walked header by header. The walk keeps a per-frame
byte offset and advances it across all frames at once (one pass per header
slot), so frames with different PDU counts, orders, and lengths are handled. For
each contained PDU id the matching payload slices are gathered into one array
and its signals are extracted with the existing extract_signal. A contained
PDU's signal start bits are relative to the PDU payload, so no offset rebasing is
needed.

_extract_can_logging() in src/asammdf/blocks/mdf_v4.py routes frames where
message.is_pdu_container is true through extract_pdus(). All other frames
keep using extract_mux().

Output: each contained PDU becomes its own channel group, created by the
existing channel-group code. extract_pdus() returns the same structure as
extract_mux() with the PDU identity in the entry key, so the caller is
unchanged apart from the routing branch.

Decoding matches canmatrix.Frame.unpack() for container frames.

Signedness fix in extract_signal

extract_signal() decoded a signed signal of standard bit width (8, 16, 32, 64)
at a non-byte-aligned offset by viewing the padded byte block as i{std_size},
which does not sign-extend from the field width. Example: an 8-bit signed field
at bit offset 1 returned 225 instead of -31. The condition now also uses
as_non_byte_sized_signed_int when the field is not byte aligned. This path is
shared with extract_mux, so the change also affects normal (non-container)
extraction of such signals.

Tests

test/test_CAN_pdu_extraction.py (offline, uses canmatrix.Frame.unpack() as
the reference):

• extract_pdus() output compared to the reference for big-endian and
  little-endian headers and for 0x00 and 0xFF padding, over frames with random
  PDU count and order.
• Full extract_bus_logging() run on a synthesized CAN-logging MF4, checking one
  channel group per contained PDU and matching sample values.
• Signed, non-byte-aligned signals (big and little endian) inside one PDU.
• A container without Header_ID/Header_DLC returns no signals.

Existing test/test_CAN_bus_logging.py (OBD2 and J1939) still passes.

Not covered

• Static PDU containers (no per-PDU header) are skipped and produce no signals.
• PDU-internal multiplexing.
• LIN container frames.
• The CAN-FD frame flag path is not exercised in tests; container frames were
  built at 64-byte width without the FD flag set.