Code generate IsExternalInit

[cloutiertyler]

clockworklabs/spacetime-docs#125 (comment)

In relation to this issue, we would like to actually just code generate a file which includes this code. That way only users who already have a definition will get an error, rather than every user who uses SpacetimeDB's Unity SDK.

We will have to document what to do in the duplicate case (that will only be documented on our website however).

We think this is preferable to the alternative though. This will presumably also require a --unity flag or something in the code generation so that it is not generated for C#.

[gefjon]

This may get a code freeze exception. @cloutiertyler is undecided.