Configurable Join Behavior for Implicit Joins

### Current Behavior
Currently, when users merge multiple connections into a single activity, Elsa treats it as a "wait all" join. This means that all connections must be completed before the merged activity is triggered. For any behavior other than "wait all," users are required to introduce an explicit Join activity and configure its Join Mode setting.

### Proposed Enhancement
To amplify Elsa's core principles of flexibility and user-centric configurability, we suggest expanding this behavior to be adaptable based on user preferences.

This configurable behavior should be available at two granularity levels:

1. **Application-wide Configuration:** This would set the default behavior for all workflows in the application. Ideal for users or organizations that have a consistent preference across all workflows.

2. **Per Workflow Definition:** This will allow users to specify join behaviors on a workflow-by-workflow basis. This is crucial for scenarios where different workflows demand varied join behaviors.

3. **Per Flowchart Activity:** This allows users to specify join behaviors on a flowchart-by-flowchart basis. This allows granular control over the implicit join behavior within the same workflow definition.

By implementing this enhancement, Elsa will empower its users with more autonomy and less complexity in designing workflow merges.

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### Discussed in https://github.com/elsa-workflows/elsa-core/discussions/4483