api!(jsonrpc): remove webxdc info from MessageObject

[link2xt]

Message.get_webxdc_info() may fail if the .xdc attachment is invalid, e.g. if it is not a ZIP file. Calling it while loading MessageObject resulted in a problem #6826 with loading the message that has Webxdc viewtype, but no valid .xdc attachment. The problem is worked around since #6866 (commit 81a6afd) by logging the error and ignoring it, but no error is returned to the JSON-RPC method caller.

I now want to modify the logging macros to depend on a tracing crate which may not be imported by other crates such as deltachat-jsonrpc (#6919), so to avoid exposing this to the caller I am making the log module private (#6918). If we keep the workaround with logging the error and ignoring it, it should be moved into the deltachat crate as a Message.get_webxdc_info_or_log_err() variant that ignores errors and then make another variant of WebxdcMessageInfo::get_for_message that ignores errors as well.

We don't ignore errors anywhere else, so a proper solution seems to move loading that may fail into a separate call so JSON-RPC calls can make it when Webxdc viewtype is detected and handle the error themselves. JSON-RPC method get_webxdc_info for this even exists already.

[WofWca]

I'll re-state that this will require modifications to clients' code (Desktop, namely).
I have not assessed exactly what modifications will need to be made.
But I think this is a more correct approach.