feat: add placeholder implementation for ModulationParameters, AntennaPattern, and VariableTransmitterParameters

[ngjunsiang]

Summary

TransmitterPdu relies on the above for a complete implementation. These three classes are a challenge because:

• their contents depends on other PDU attributes, e.g. modulationType, antennaPatternType, recordType, ...
• they read a variable number of bytes from the stream

This PR sets up a skeleton for handling such variable-size, variable-type records.

• adds ModulationParametersRecord and AntennaPatternRecord base classes
• adds placeholder implementation for VariableTransmitterParameters
• adds UnknownRadio and UnknownAntennaPattern placeholder concrete classes that read the appropriate number of bytes into a placeholder attribute

These changes enable TransmitterPdu to finally read the stream completely.

Future work

• add more concrete classes (radio records, antenna patterns, VTPs, ...) following this skeleton. The concrete classes extend the base classes to facilitate type annotation
• organize the records namespace as more types of records are added
• extend the TransmitterPdu tests

Issues

• Fix TransmitterPDU data appears to be misinterpreted.  #62

[leif81]

@ngjunsiang need more time or ready to merge?

[ngjunsiang]

@leif81 it's ready 👍 will add future changes in a separate PR to keep this one easy to review