Allow custom board implementations for sx1261/2 and sx1276/7/8/9

[ceekdee]

Add custom board and chip types to allow users to implement external RadioKind and InterfaceVariant traits for a custom (perhaps proprietary) sx1261/2-based or sx1276/7/8/9-based LoRa board. Use those implementations underneath the standard LoRa API. For example:

• let iv = ExternalInterfaceVariantImpl::new(...params...)
• LoRa::new(ExternalRadioKindImpl::new(BoardType::CustomSx1261_2, spi, iv), ...other_params...)

This PR partially addresses Issue #13.

Complete freedom to use a LoRa chip unknown to lora-phy will require a new major release, since the implementations of modulation and packet parameter creation contain logic specific to the radio kind, but is not currently implemented within the RadioKind trait. An update to the RadioKind trait will necessitate a new major release.

[ceekdee]

Please let me merge this PR myself, once we agree on the solution. I need to consider the order of releases to crates.io.