App-services crate / UDL file

[bendk]

Let's create an app-services crate for code that is shared between multiple components. This has a couple benefits:

• We have several small crates that we want to expose via UniFFI (crashtest, shutdown, rc_log). If we have an app-services crate, we can add them there instead of having to create a new UDL file, integrate it into the Android and iOS build systems, etc. Maybe the encryption code also fits into this category.
• We have several types that are shared between components (DeviceType and other types from sync15 and sync15-traits, SqlInterruptHandle, etc). If these existed in a shared UDL file, then we could define them as external types in the UDL file and not have to repeat the definitions. (prior work)

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