WIP: feat/show upload hash --> improve typehints around ShowSpecification

[vasarhelyi]

This PR parses an optional show hash that is coming from latest Live (https://github.com/skybrush-io/live/tree/feat/show-upload-hash branch) in connection with the new collective RTH support.
It is backwards compatible, if no hash is provided it defaults to None.

We abandoned the new show hash param, but some typehint updates still remained in this PR so can be merged now, it won't alter functionality.

[ntamas]

Before we go deeper into the review process: I don't understand a few conceptual things:

What's the purpose of the show hash here?

Is the purpose to be able to distinguish between different upload sessions on the server side? If so, then we don't need a hash because that could be identical for different upload sessions of the same show. We need a "session ID" instead, which can be an arbitrary opaque string generated on Live's side with nanoid or anything else.

Or, is the purpose to be able to distinguish whether the show being uploaded is still the same show as the previous one so we can collect the RTH plan timestamps? In that case, we don't need a hash either -- we need a unique show ID, provided by the design software and most likely already pre-generated in the .skyc file, and the perfect place to embed this would be the show specification itself, no need for a separate argument.

Or, do we want to distinguish between uploads of the same show with different geofence or safety configurations? In that case, the hash can still belong to the show specification as the geofence configuration and other safety settings are also there.

So, all in all, for me it seems that either we don't need a separate argument (and we could just embed the ID in the show specification), or if we do, then we need a session ID, not a hash.

[vasarhelyi]

@ntamas I removed the proposed show hash functionality, some typehint improvements remained, please merge.