identify potential uses cases for prefill and decide whether to support them

[tshuli]

Related to #498

For discussion on whether to expand prefill to support the following use cases

• Singpass forms; currently the query params are lost on singpass redirect
• Non-english characters; currently not supported due to sanitizing input
• Other field types. Although short text field already accepts string (i.e. can input any value), expanding to other field types will allow you to apply more types of custom validation (e.g. number validation). Note that short text custom validation already works with prefill

[tshuli]

@syan-syan I think these are the main use cases discussed so far, for your further assessment

[tshuli]

Shifted discussion to #498