Library for developing a starlark function

[phanimarupaka]

We need a starter kit so that users can write a simple function by adding few lines of code. Eventually this will be provided by sdk but we need something now to help users.

[phanimarupaka]

https://github.com/GoogleContainerTools/kpt-functions-catalog/pull/274

[mengqiy]

If you want to choose starlark in the starter kit, we have some examples that are not currently living in tha catalog repo: https://github.com/GoogleContainerTools/kpt/issues/1563#issuecomment-833701352

[mengqiy]

With GoogleContainerTools/kpt-functions-catalog#570 merged, we can consider to provide our own starlark library and allow users to load it in the starlark function. The library should improve the function authoring experience. It should provide some useful helper functions.