Event Registration: allow default Price Set values from the URL

[mlutfy]

Overview

Contribution Pages allow site-admins to share an URL with arguments such as price_123=22 so that the form has specific default values. This was is not currently possible with events.

This PR implements the same solution for default values for public Event registration.

See: https://civicrm.stackexchange.com/questions/40181/query-string-to-pre-select-price-field-for-events

Technical Details

Based on the code from Contribution Pages. Admittedly some copy-pasting.

Comments

It does not work for multiple-participant registration (it only sets on the first participant).

[civibot]

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[andyburnsco]

I've tested this on Civi 5.35.2 for radio, checkbox and text/numeric quantity field types and it works perfectly.

[seamuslee001]

Merging as per the testing