Better header insertion handling

[rylev]

Worked on with @yoshuawuyts 🎉

Better handling of how we add headers to requests.

Before

azure_core::headers::add_optional_header2(&self.consistency_level, &mut request)
   .with_context(ErrorKind::DataConversion, || {
      format!(
        "could not encode '{:?}' as an http header",
        self.consistency_level
       )
   })?;

After

request.insert_header(&self.consistency_level)?;

This PR is best reviewed commit by commit.

[yoshuawuyts]

🎉

[cataggar]

Wow, what an improvement here!