Want API to report supported speeds for a module

[bnaecker]

Today, we report the datapath information for a transceiver, which includes things like the specification compliance code for SFF-8636 modules or the media interface ID for CMIS modules. It would be nice to include an API for getting the list of supported speeds from a datapath. For example, for 100G CWDM4 modules, we'd return [100_000_000_000] or something similar. For multi-speed modules, that might have many entries.