issue #7441 : Add Enter Field Mapping button to Update transform

[mattcasters]

What changes were proposed in this pull request?

This closes #7441

Adds an Enter field mapping button to the Update transform dialog (Fields tab), similar to Table Output and Insert/Update. This makes it much easier to map stream fields to table columns when configuring updates.

Changes:

• Add Enter field mapping button next to Get update fields in UpdateDialog
• Open EnterMappingDialog with stream fields and target table fields, preserve existing mappings, and write results back to the update fields table
• Implement UpdateMeta.getRequiredFields() for consistency with related transforms
• Add English i18n messages for the button and mapping error/confirm dialogs

What was tested?

• ./mvnw -pl plugins/transforms/update -am compile -DskipTests
• ./mvnw -pl plugins/transforms/update test (5 tests, all passing)

How can a reviewer test this PR?

1. Open a pipeline with an Update transform connected to a previous transform
2. Configure a database connection and target table
3. Open the Update dialog → Fields tab
4. Click Enter field mapping
5. Map stream fields to table columns and confirm the update fields table is populated
6. Verify Cancel on the mapping dialog leaves the table unchanged, and Cancel on the transform dialog still discards edits