Segregate and deprecate avro reflective records support

[jto]

This PR is an attempt to discourage the usage of reflective serialization in Avro and avoid its accidental use.

It separates "normal" IOs on GenericRecord and avro magic reflection based serialization. Back compatibility is preserved but a deprecation warning is issued.

[nevillelyh]

LGTM it's backwards compatible right?

[jto]

Yes

[codecov]

Codecov Report

Merging #2300 into master will increase coverage by <.01%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #2300      +/-   ##
==========================================
+ Coverage   71.25%   71.25%   +<.01%     
==========================================
  Files         240      240              
  Lines        7743     7766      +23     
  Branches      353      352       -1     
==========================================
+ Hits         5517     5534      +17     
- Misses       2226     2232       +6

Impacted Files	Coverage Δ
...m/spotify/scio/avro/syntax/SCollectionSyntax.scala	100% <100%> (ø)	⬆️
.../src/main/scala/com/spotify/scio/avro/AvroIO.scala	91.76% <100%> (+0.97%)	⬆️
...ro/src/main/scala/com/spotify/scio/avro/taps.scala	42.3% <50%> (+2.3%)	⬆️
...m/spotify/scio/avro/syntax/ScioContextSyntax.scala	83.33% <66.66%> (-16.67%)	⬇️
...main/scala/com/spotify/scio/util/BloomFilter.scala	93.47% <0%> (-0.73%)	⬇️
...com/spotify/scio/testing/SCollectionMatchers.scala	96.77% <0%> (-0.07%)	⬇️

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[jto]

@nevillelyh Can we merge this ?