refactor: change host function API and hide GuestRuntime

[mhmd-azeez]

I wanted to change these before hitting 1.0 for the go-sdk:

• I think we should hide HostFunction creation behind a function so that we have flexibility of adding new function callbacks in the future
• And I think GuestRuntime is an implementation detail and doesn't need to be public

[mhmd-azeez]

@zshipko @bhelx @nilslice will merge this PR if no one has any objections

[nilslice]

I think this looks great -- will wait for one other review though.. someone a little closer to the previous Host Functions impl would be ideal!

[zshipko]

Looks good!

[mhmd-azeez]

cc @k33g you might need to update your demo for your talk

[k33g]

@mhmd-azeez thanks for the notification, I will have a look :)