Fix build with fmt 11

fmt 11 enforces that fmt::formatter<..>::format() should be const.
otherwise the tree does not build:

```
/usr/include/fmt/base.h:1392:29: error: passing ‘const fmt::v11::formatter<std::optional<mtx::http::ClientError> >’ as ‘this’ argument discards qualifiers [-fpermissive]
 1392 |     ctx.advance_to(cf.format(*static_cast<qualified_type*>(arg), ctx));
      |                    ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

so let's mark the `format()` with `const` specifier.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

This is required by the spec, however no such condition is actually
present in the rules... To work around that, we just add our own, but we
should write an MSC to fix that eventually.

add default_payload to PusherData struct

URL preview support

Add function to report content