Extend Invitation Toolset support

[defstat]

Describe the change
Enhancement of the Invitation API and General Invitation Toolset

Invitation API Changes:

• Role Assignment: Extend the Invitation API to support Role Assignment for both existing and new users.
• Validation Handling: Implement Laravel's Validation API to improve the handling of validation errors in the Invitation API.
• API Resources: Use Laravel API Resources to return the Invitation object in a consistent format.

General Invitation Toolset Changes:

• Invitation Structure: Change the invitation structure so that each invitation is placed in its own folder.
• Code Reduction: Refactor the invitation toolset to minimize code. Specifically, move the accept/decline logic to dedicated handlers (e.g., API Handlers) to reduce complexity in the invitation logic.

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PRs

PKP-LIB: #10472

OJS: pkp/ojs#4448
OMP: pkp/omp#1708
OPS: pkp/ops#778

[defstat]

@asmecher everything merged

[asmecher]

Excellent, thanks! 🎉