Allow child themes to modify the event parameters for the `formSubmission` event

[stduerre]

Brief summary of task:

When a form is submitted, a formSubmission event is pushed to dataLayer. Some NROs have web analytics setups that differ from the global one and need additional parameters to be sent with the event. Adding a filter hook for the event data would solve this.

Objectives of task:

• Generally: Enable NROs to modify the data that's sent with a formSubmission event.
• Specific use case: Enable GP Switzerland to add newletter subscription data and and gp_user_id to the formSubmission event

Relevant documentation:

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Other resources:

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Estimated length of task:

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Timeline for task:

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Dependencies on other work:

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Example of similar work:

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Help:

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[comzeradd]

Fixed at greenpeace/planet4-master-theme/pull/2297