Added support to read Avro files' metadata asynchronously

[jorgecarleitao]

This is one of the two steps necessary to support full async reading for Avro.

Avro is often used as a streaming format and thus supporting async has more benefits than e.g. Parquet.

Supporting async for the remainder of the file will be done on a separate PR.

The main benefit of this PR is that query engines that wish to scan a remote Avro file can do so in an async manner.

As with all other IO modules, the async code:

• is under a feature gate (io_avro_async)
• performs minimal CPU work (it still needs to parse a json blob)

[codecov]

Codecov Report

Merging #614 (3903150) into main (9d4107c) will decrease coverage by 0.05%.
The diff coverage is 69.16%.

@@            Coverage Diff             @@
##             main     #614      +/-   ##
==========================================
- Coverage   79.61%   79.55%   -0.06%     
==========================================
  Files         403      408       +5     
  Lines       24838    24856      +18     
==========================================
  Hits        19775    19775              
- Misses       5063     5081      +18     

Impacted Files	Coverage Δ
src/io/avro/mod.rs	0.00% <0.00%> (ø)
src/io/avro/read_async/mod.rs	0.00% <0.00%> (ø)
src/io/avro/read/mod.rs	81.25% <33.33%> (-1.42%)	⬇️
src/io/avro/read/block.rs	70.58% <70.58%> (ø)
src/io/avro/read/decompress.rs	80.64% <80.64%> (ø)
src/io/avro/read/header.rs	100.00% <100.00%> (ø)
src/io/avro/read/util.rs	95.00% <100.00%> (+9.89%)	⬆️
tests/it/io/avro/read.rs	100.00% <100.00%> (ø)
tests/it/io/avro/read_async.rs	100.00% <100.00%> (ø)
src/io/parquet/read/nested_utils.rs	78.43% <0.00%> (+0.98%)	⬆️

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