eds: Support custom_options for unknown EDS fields

[bizfsc]

Supersedes #615.

When importing an EDS/DCF file, any key that is not part of the CiA 306 standard field set is now collected into a custom_options dict on the object. This allows applications to round-trip vendor-specific metadata without losing it.

Changes

• ODVariable, ODRecord and ODArray gain a custom_options: dict attribute (default {}).
• New _STANDARD_OPTIONS set in eds.py lists all keys that are parsed explicitly (including ObjFlags and Denotation which are handled by a separate PR).
• New _get_custom_options(eds, section) helper collects every key that is not in _STANDARD_OPTIONS.
• custom_options is populated for top-level VAR/DOMAIN, ARRAY and RECORD objects as well as for sub-variables via build_variable().
• export_variable() and export_record() write custom_options back to the EDS file, preserving all unknown fields through a round-trip.
• Imports aligned with the master style (from canopen.objectdictionary import …, objectcodes).
• Local DOMAIN/VAR/ARR/RECORD constants removed; replaced with objectcodes.DOMAIN/VAR/ARRAY/RECORD.

Tests

• test_reading_custom_options — VAR with Category=Motor, Offset=100
• test_custom_options_standard_keys_excluded — standard keys absent from custom_options
• test_custom_options_empty_for_standard_object — heartbeat time has {}
• test_custom_options_record — RECORD-level custom_options
• test_roundtrip_custom_options — full EDS export/import cycle preserves values
• test_roundtrip_custom_options_not_duplicated_as_standard — standard keys stay out after round-trip

[codecov]

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