Add generic invalid_value_action parameter to all Neosync transformers

[dp-dandy]

Is your feature request related to a problem? Please describe.
Currently, only the neosync_email transformer has an invalid_email_action parameter to handle null/invalid input values. Other Neosync transformers (neosync_firstname, neosync_lastname, neosync_fullname, neosync_string) lack this capability, which causes:

1. NOT NULL constraint violations when transformers return null for invalid/null input data
2. Inconsistent behavior across the Neosync transformer family
3. Configuration complexity where some transformers handle edge cases and others don't
4. Data pipeline failures requiring workarounds with template transformers

Describe the solution you'd like
Add a generic invalid_value_action parameter to all Neosync transformers with consistent behavior and values:

invalid_value_action: generate  # generate, passthrough, null, reject

Parameter Values:

• generate (default): Generate a valid transformed value even for null/invalid input
• passthrough: Keep the original value unchanged
• null: Explicitly set to null
• reject: Throw an error and stop processing

Transformers that should support this:

• neosync_email (already has invalid_email_action - could be aliased to invalid_value_action)
• neosync_firstname
• neosync_lastname
• neosync_fullname
• neosync_string

Example configuration:

transformations:
  table_transformers:
    - schema: public
      table: users
      column_transformers:
        first_name:
          name: neosync_firstname
          parameters:
            preserve_length: true
            invalid_value_action: generate
        last_name:
          name: neosync_lastname
          parameters:
            preserve_length: true
            invalid_value_action: generate
        email:
          name: neosync_email
          parameters:
            preserve_length: true
            preserve_domain: true
            invalid_value_action: generate  # replaces invalid_email_action

Describe alternatives you've considered

1. Current workaround: Using template transformers with conditional logic

   name: template
   parameters:
     template: >
       {{ if and (ne .GetValue nil) (ne .GetValue "") }}
         {{ faker "first_name" }}
       {{ else }}
         FirstName
       {{ end }}

   Problems: Verbose, inconsistent, requires template knowledge

2. Transformer-specific parameters: Adding invalid_firstname_action, invalid_lastname_action, etc.
   Problems: Increases configuration complexity, inconsistent naming

3. Global default handling: Setting a global policy for all transformers
   Problems: Less flexible, doesn't allow per-transformer customization

Additional context
This enhancement would:

• Improve consistency across all Neosync transformers
• Reduce configuration complexity with standardized parameter naming
• Prevent data pipeline failures from NOT NULL constraint violations
• Future-proof any new Neosync transformers added to pgstream
• Maintain backward compatibility by keeping invalid_email_action as an alias

Benefits for common use cases:

• Database replication: Handles missing/corrupt data gracefully during snapshots
• Data anonymization: Ensures all sensitive fields get properly masked even with dirty data
• Testing environments: Provides realistic test data even when source data has gaps
• Compliance: Guarantees consistent data protection across all transformed fields

Implementation suggestion:
The generic invalid_value_action parameter could be implemented at the pgstream integration layer, similar to how invalid_email_action currently works, providing consistent null/invalid value handling across all Neosync transformers without requiring changes to the underlying Neosync library.