fix(snippet): update custom field snippet

[Benunc]

User Story

As a Give developer, I want an up-to-date example of how to programmatically add a custom field so that it ends up in post meta like any other custom field.

Acceptance Criteria

• The add_post_meta call has been changed to give_update_payment_meta in the snippet.
• The snippet has been tested and confirmed that its post meta is stored correctly in the database.
• The custom field is visible in Donation Details screen.
• The custom field appears on Donation Confirmation page.
• The custom field is available for export in Tools > Exporter
• The custom field is available for Email Tags
• Support team has been notified that the snippet is ready.

Related

• Snippet URL - https://github.com/WordImpress/Give-Snippet-Library/blob/master/form-customizations/custom-fields-plugin.php
• Outdated doc - https://givewp.com/documentation/developers/how-to-create-custom-form-fields/
• https://github.com/WordImpress/Give-Docs/issues/166

Tasks

1. Review the [outdated documentation(https://givewp.com/documentation/developers/how-to-create-custom-form-fields/).
2. Update the snippet to work with Give 2.0+
3. Notify support in https://github.com/WordImpress/Give-Docs/issues/166 so the doc can be updated.

[kevinwhoffman]

Slack Call Summary

Participants: @raftaar1191 @kevinwhoffman
Topic: Discuss the scope of the custom field snippet and where it needs to appear
Result: We have updated the acceptance criteria to reflect where the custom field should appear.

@mathetos @Benunc Would you mind reviewing the Acceptance Criteria from a support perspective in case we have overlooked anything? This updated snippet will become the basis for the updated doc. Thanks.

[Benunc]

It looks great to me. I'm happy to test the snippet once it's ready.

[mathetos]

@kevinwhoffman Same. That criteria is great.