Session file support for bulk migration using JSON node

[Deep1998]

Contains the session file support in the bulk template using JSON node as the standard format. If we decide to finalise avro, we will update this PR with avro support.

This PR supports:

• Name transformations, synth PK handling and dropped columns using session file.
• Almost all mysql data types except MySQL YEAR and BIT.

This PR does not support:

• Sharding support (shard id population)
• Transformation context

[codecov[bot]]

Codecov Report

Merging #1400 (fe123de) into main (f8653ef) will increase coverage by 0.07%. Report is 11 commits behind head on main. The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1400      +/-   ##
============================================
+ Coverage     38.92%   38.99%   +0.07%     
+ Complexity     2761     2759       -2     
============================================
  Files           743      745       +2     
  Lines         41939    41992      +53     
  Branches       4516     4527      +11     
============================================
+ Hits          16324    16375      +51     
  Misses        24127    24127              
- Partials       1488     1490       +2     

Components	Coverage Δ
spanner-templates	52.46% <100.00%> (+0.28%)	:arrow_up:
spanner-import-export	65.22% <ø> (-0.03%)	:arrow_down:
spanner-live-forward-migration	57.20% <100.00%> (+0.97%)	:arrow_up:
spanner-live-reverse-replication	38.44% <100.00%> (+0.91%)	:arrow_up:
spanner-bulk-migration	59.40% <100.00%> (+1.27%)	:arrow_up:

Files	Coverage Δ
...eport/v2/spanner/migrations/schema/LazyMapper.java	100.00% <100.00%> (ø)
.../spanner/migrations/schema/SessionBasedMapper.java	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes