feat: optimize FieldMapping init func & optimize convertTo when use '…

[meguminnnnnnnnn]

…only From' field mapping

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[github-actions]

📊 Coverage Report:

File coverage threshold (20%) satisfied:	PASS
Package coverage threshold (30%) satisfied:	PASS
Total coverage threshold (75%) satisfied:	PASS
Total test coverage: 84.1% (3286/3907)

[github-actions]

⚠️ Breaking API Changes Detected

This pull request contains breaking API changes that may affect compatibility:

uncompatible api changes found: 
github.com/cloudwego/eino/compose
  Incompatible changes:
  - From: removed
  - FromTo: removed
  - To: removed
  Compatible changes:
  - FromField: added
  - MapFields: added
  - ToField: added

Required Actions:

1. Please review these changes carefully
2. Update the user documentation to reflect these API changes
3. Add a note in the changelog about these breaking changes