refactor: changes to aggregate::Repository, event::Store, documentation and more

[ar3s3ru]

In the scope of a closer v0.5.0 release, this PR introduces quite some refactorings to different APIs, which are still experimental:

refactor: generalize event::StreamVersionExpected into version::Check

version::Check is a more general phrasing and naming for optimistic locking, yet expressing quite more precisely its role.

feat: add new aggregate::test::Scenario

It is now possible to write test assertions on aggregate::Root methods, rather than having that only available through a command::test::Scenario.

refactor(aggregate::repository): split Repository traits once again, use concrete error type

After trying the approach of having an unified aggregate::Repository trait, and use an AnyRepository with Error = anyhow::Error (so, opaque) for better usage in trait objects, we've reverted back to Interface Segregation:

1. Have two separate interfaces -- Saver and Getter -- and a Repository one, which extends both,
2. The errors returned by those are concrete types, using thiserror, rather than using an associated type to abstract over it.

While this has required the introduction of opaque errors with anyhow::Error in the concrete error types, this should provide a better ergonomics for consumers of this API.

refactor(event::store): move symbols to this module, add AppendError concrete type

The event::Store follows the same approach as aggregate::Repository now: split interfaces, concrete error types. The only exception being Streamer::stream, where it seems to be fine to abstract over the error type returned (for now).

refactor(eventually-macros): remove derive(Message), as it requires better design

#[derive(Message)] requires better design; we've implemented that with a specific idea in mind, but that's very limiting and wouldn't be used most of the time.

We remove it for now while we collect some ideas on how it should work.

chore: add vscode setting for rust-analyzer to use all features

Finally rust-analyzer under VSCode will compile and check the workspace using all feature flags.

docs(eventually): add missing documentation

There was A LOT of missing documentation. This PR addresses some of the missing stuff.
It's not great by any means, we'd likely need some dedicated effort before releasing.

refactor: split Serde trait into separate ones, implement serde::Convert and serde::ProtoJson

After some production usage, we've collected some different ideas on how this trait should work, and have implemented that following the previous concepts: split methods/traits, concrete error types.

chore: update all versions

Too many open dependabot PRs, this will take care of them all at once (but syn, which is a major change).

[codecov]

Codecov Report

Attention: Patch coverage is 62.53687% with 127 lines in your changes are missing coverage. Please review.

Project coverage is 84.89%. Comparing base (be239c7) to head (4a4977e).

Files	Patch %	Lines
eventually/src/aggregate/test.rs	0.00%	61 Missing ⚠️
eventually/src/serde.rs	23.40%	36 Missing ⚠️
eventually/src/aggregate/mod.rs	75.51%	12 Missing ⚠️
eventually-postgres/src/aggregate.rs	86.66%	6 Missing ⚠️
eventually/src/tracing.rs	0.00%	5 Missing ⚠️
eventually-postgres/src/event.rs	93.61%	3 Missing ⚠️
eventually/src/event/store.rs	87.50%	3 Missing ⚠️
eventually/src/aggregate/repository.rs	97.82%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #289      +/-   ##
==========================================
- Coverage   91.53%   84.89%   -6.64%     
==========================================
  Files          17       15       -2     
  Lines        1158     1225      +67     
==========================================
- Hits         1060     1040      -20     
- Misses         98      185      +87     

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