Clean up unions

[jamescourtney]

• remove switch methods (Accept is better anyway)
• Reduce number of built-in FlatBufferUnion types -- this expodes AOT codegen sizes. Instead, add interfaces for all necessary IFlatBufferUnion types and rely on FBS compiler to cons up other unions as necessary.

[codecov[bot]]

Codecov Report

Merging #314 (de1b13e) into main (23f1960) will decrease coverage by 0.05%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #314      +/-   ##
==========================================
- Coverage   95.89%   95.83%   -0.06%     
==========================================
  Files         114      114              
  Lines        7992     7905      -87     
  Branches      761      751      -10     
==========================================
- Hits         7664     7576      -88     
  Misses        222      222              
- Partials      106      107       +1     

Impacted Files	Coverage Δ
...FlatSharp.Compiler/SchemaModel/UnionSchemaModel.cs	96.52% <ø> (-1.31%)	:arrow_down:
src/FlatSharp/FlatBufferSerializer.cs	89.00% <0.00%> (-1.00%)	:arrow_down:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 23f1960...de1b13e. Read the comment docs.