Implementing custom types

[jhugman]

According to The Big O of Code Reviews, this is a O(n) change.

This adds the custom-types example fixture allowing you to write handwritten conversion logic into the generated typescript.

To enable this, we add a:

• --toml argument to the cargo xtask run command line for testing,
• --config argument to the ubrn generate bindings; this is to keep it the same as other uniffi-bindgen backends,
• uniffiToml property in bindings section of the config.yaml files used elsewhere.