feat(nx-plugin-openapi): add support for --template-dir option

[matteomonti-bo]

Add support for --template-dir configuration via templateDir option.

Fixes: #30
Relates to: #69

[matteomonti-bo]

Hi @trumbitta!
I'm trying to add an e2e test for this feature, but I'm struggling to figure out how to create a mock template folder in the test project (tmp/nx-e2e/proj).
I can't find a configuration file for the proj project, where I can set the assets configuration in the build phase; but I'm not sure this is the best way to handle this.
Could you please advise on how you would prefer it to be done? Thanks!