build(scripts): move client generation to its own npm script

[JadenSimon]

Problem

No standalone script to generate service client type definitions

Solution

Split the command out into generateClients and remove it from postinstall

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jpinkney-aws]

So now that generating service clients is out of the postinstall, is there a chance that we/any contributor might forget to do it? Would that break the build and have people wondering what they might have to do to correct the build?

[JadenSimon]

So now that generating service clients is out of the postinstall, is there a chance that we/any contributor might forget to do it? Would that break the build and have people wondering what they might have to do to correct the build?

It's still ran before any build so no. Code generation for building the package should really not be in postinstall.