feat: add trasformers from/to for DataFlowStartMessage

[wolf4ood]

What this PR changes/adds

• Adds trasformers from/to for DataFlowStartMessage
• Replaces transferType with flowType as described docs: add documentation about mapping the format #3924
• Moves FlowType into core-spi

Why it does that

data plane signaling

Linked Issue(s)

Part of #3934

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.45455% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 73.28%. Comparing base (7f20ba5) to head (d40363b).
Report is 110 commits behind head on main.

Files	Patch %	Lines
...pi/types/domain/transfer/DataFlowStartMessage.java	60.00%	2 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3940      +/-   ##
==========================================
+ Coverage   71.74%   73.28%   +1.53%     
==========================================
  Files         919      948      +29     
  Lines       18457    19096     +639     
  Branches     1037     1082      +45     
==========================================
+ Hits        13242    13994     +752     
+ Misses       4756     4642     -114     
- Partials      459      460       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.