Migrated from avro-rs to avro-schema

[jorgecarleitao]

This PR replaces avro-rs by avro-schema (authored by me).

Background

While working on reading from Avro some months ago, and now on #690, I noticed 4 issues with avro-rs, some of which are blocking:

1. I was unable to create a Schema::Union since both its properties and the constructor are private :/
2. I identified a bug which was causing avro-rs to write schemas incorrectly
3. avro-rs drags dependencies and code to deserialize from Avro, but we do not use it since we implement deserializers directly to arrow
4. avro-rs is unmaintained (its new home seems so as well), and some of my PRs there are not being merged or dragged upstream

This PR

Given the above, I wrote a new crate, avro-schema, containing the declaration of the Avro schema (as per spec) in Rust. This hopefully offers more flexibility and unblocks #690

The other added bonus are:

1. The create only depends on serde and serde_json
2. The crate is quite small (<800 LOC) and #![forbid(unsafe_code)]
3. addresses all the issues mentioned above

[codecov]

Codecov Report

Merging #692 (dc1784d) into main (f64339c) will decrease coverage by 0.04%.
The diff coverage is 46.51%.

@@            Coverage Diff             @@
##             main     #692      +/-   ##
==========================================
- Coverage   69.77%   69.73%   -0.05%     
==========================================
  Files         303      303              
  Lines       16855    16865      +10     
==========================================
- Hits        11761    11760       -1     
- Misses       5094     5105      +11     

Impacted Files	Coverage Δ
src/io/avro/mod.rs	0.00% <ø> (ø)
src/io/avro/read/util.rs	95.00% <ø> (ø)
src/io/avro/read/schema.rs	53.68% <40.00%> (+1.30%)	⬆️
src/io/avro/read/header.rs	84.61% <60.00%> (-15.39%)	⬇️
src/io/avro/read/deserialize.rs	66.66% <100.00%> (-5.99%)	⬇️
src/io/avro/read/mod.rs	81.25% <100.00%> (ø)
src/io/avro/read_async/metadata.rs	95.00% <100.00%> (ø)

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