Add API to read page content

It seems that the card value attribute could only be referenced and is write only.
If I'd like to read from the value (or aux_value) attribute, it returns a reference object instead of a plain value.
You may doubt the necessity of such reverse access.
The possible intent may be:
1. the other separate client needs to read current value from the running card
2. the same client needs to access the aux_value (which is the fixed value set during card creation) to calculate live data, say progress.

No files, tests, or entry points are named. Start by locating the API definitions for card value and aux_value, then verify how reads are represented; done means both attributes can be read as plain values for the stated client use cases.

Written by the indexing model from the issue text.