Switch DatastreamToSpannerDataTypeIT from JSON to Avro

[manitgupta]

The current ITs use JSON data from Datastream which is not used in actual migrations.

Switching ITs to use avro files from Datastream instead of JSON to more closely resemble a production scenario. This involves generating all the test data and verifying that assertions are correct.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (main@ace900d). Click here to learn what that means.
Report is 35 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1525   +/-   ##
=======================================
  Coverage        ?   40.08%           
  Complexity      ?     2804           
=======================================
  Files           ?      754           
  Lines           ?    42710           
  Branches        ?     4587           
=======================================
  Hits            ?    17121           
  Misses          ?    24110           
  Partials        ?     1479           

Components	Coverage Δ
spanner-templates	56.75% <ø> (?)
spanner-import-export	65.59% <ø> (?)
spanner-live-forward-migration	61.31% <ø> (?)
spanner-live-reverse-replication	42.63% <ø> (?)
spanner-bulk-migration	72.05% <ø> (?)

[darshan-sj]

LGTM