runtime: add support for structure group with base type that does not require known values

[joachimmetz]

Some formats have a base type, eg. lnk data blocks https://github.com/libyal/liblnk/blob/main/documentation/Windows%20Shortcut%20File%20(LNK)%20format.asciidoc#6-extra-data, that have a base type that can be read without specific support for identifying values, versus eg. bsm token, https://github.com/libyal/dtformats/blob/main/documentation/Basic%20Security%20Module%20(BSM)%20event%20auditing%20file%20format.asciidoc#1-overview, that requires a known token type

to consider:

• add option to definition indicate identifying value is required? (full versus partial base type?)
• derive if identifying value is required from base type?