Add image reject filters

[mossmann]

As seen to the right of the RFFC5072 mixers in the block diagram: https://github.com/greatscottgadgets/lab-notes/tree/main/project-reports/2023-06-02-urti-progress-report#mainboard-block-diagram

This is very similar to the filter section in HackRF One where we have two SP3T switches that provide three paths: high-pass, low-pass, and band-pass. The band-pass path could be straight-through without an actual filter.

We could use off-the-shelf filters as in HackRF, or we could design lumped element filters. We might consider chaining two or more filters in order to achieve a sharper cut-off.

Potentially we could add more paths with tighter filters, increasing the ability to filter out additional mixer products and out-of-band blocker signals, but three paths is our minimum requirement.

A HackRF-like design would be a good place to start.