Investigate a new uniffi command that can spit out all the generated headers for a uniffi-ed crate

[skhamis]

From application-services build-xcframework.sh: https://github.com/mozilla/application-services/blob/e1abe5ad61983f00a8abf1b477afa5470876e484/megazords/ios-rust/build-xcframework.sh#L136-L139

Excerpt:

# TODO: it would be neat if there was a single UniFFI command that would spit out
# all of the generated headers for all UniFFIed dependencies of a given crate.
# For now we generate the Swift bindings to get the headers as a side effect,
# then delete the generated Swift code. Bleh.

Right now we also have to manually call each udl and call the uniffi-bindgen generate -- this command could potentially find all udls? In any case, creating this for tracking purposes.

┆Issue is synchronized with this Jira Task ┆Issue Number: UNIFFI-93