It looks like the True Pundit article (linked above) is the original source for this claim, which is sourced as "according to State Department sources". Then there was a wikileaks email (https://wikileaks.org/clinton-emails/emailid/1099) which has the subject "an SP memo on possible legal and nonlegal strategies re wikileaks", but which doesn't actually mention the strategies and only says "The result is the attached memo, which has one interesting legal approach and I think some very good suggestions about how to handle our public diplomacy". The attachment is not provided as far as I can tell.

IMO, it was likely a joke.

I agree that it sounded like a joke. However the article says she pushed ahead discussing it after the laughter ended. Given that the source didn't want to be identified it makes it rather hard to verify.

There's just absolutely no way that Hillary Clinton was seriously proposing a drone strike on the on the Ecuadoran Embassy in London.

She allegedly said this in 2010, two years before Assange entered the Ecuadorian Embassy.