Allow InputBinding.receiveMessage to be async

[wch]

This is motivated by rstudio/bslib#872.
It allows an input binding's receiveMessage() method to be synchronous or asynchronous, similar to OutputBinding.renderValue().

[wch]

It might make sense to take this PR and #3929, which makes renderContent synchronous again, and adds back renderContentAsync.

Although this PR solves the specific issue we have with bslib (when paired with rstudio/bslib#874), there are potentially cases where people have written JS code which calls renderContent and then immediately tries to do something with the new content. Changing renderContent to async could break that code. Although those cases could generally be easily fixed to work with async, it would still be a breaking change. And since we have renderHtml/renderHtmlAsync and renderDependencies/renderDependenciesAsync, it would make sense to also have renderContent/renderContentAsync.

[cpsievert]

It might make sense to take this PR and #3929

Yea, I still think we should take #3929