Code cleanups prior to stabilizing protobufs feature

[cdisselkoen]

Describe the improvement you'd like to request

The code for the protobufs experimental needs several small cleanups before stabilization. At a minimum,

• resolving all PANIC SAFETY comments that today just explain "it's an experimental feature". Panics might be acceptable in experimental features but are not for stabilized features.
• resolving the TODO introduced by Expose protobufs in public API #1345 (see Expose protobufs in public API #1345 (comment))

Describe alternatives you've considered

No response

Additional context

No response

Is this something that you'd be interested in working on?

• 👋 I may be able to implement this internal improvement
• ⚠️ This feature might incur a breaking change

[john-h-kastner-aws]

we also need to carefully consider what implementation details the protobuf format effectively makes public. E.g.: #1345 (comment)

The protobuf format is based on our internal structures rather than our external data formats. This is fine for diff testing, but not necessarily for public consumption. The validator structures in particular contain a lot of details that users shouldn't be aware of.

[john-h-kastner-aws]

Linking #1286 and #1352

[john-h-kastner-aws]

Another note #1366 (comment)

[cdisselkoen]

We will also need a longer-term solution for the docs.rs build, so as to not require protoc be available on the builder. (The short-term solution is #1434, but that will not work once protobufs are stabilized.)

One possibility is that we can have the build script be aware of when we're building for docs purposes (I think this is #[cfg(docs)] or something similar), and not run protoc. We should be able to ensure we have the same interface for documentation purposes, even without the generated Rust code, eg by replacing function bodies with unimplemented! which doesn't matter during a docs build.

[john-h-kastner-aws]

Linking another comment #1506 (comment)

current entities protobuf assume TC is already computed. We probably want to include a flag to allow the parser to compute TC