Turn on explicit-module-boundary-types

Why

Explicit types for function return values and arguments makes it clear to any calling code what is the module boundary's input and output. Adding explicit type annotations for those types can help improve code readability. It can also improve TypeScript type checking performance on larger codebases. Also it helps to avoid mistakes on early stage, because after refactoring the return type of function may not be the same as callers expect

What

Enable https://typescript-eslint.io/rules/explicit-module-boundary-types

This is a hard migration, so very likely we will need to do it iteratively, per package, per app and with separate eslint config

Start by reviewing the ESLint configuration for each package or app and the explicit-module-boundary-types rule documentation. Enable the rule incrementally with separate ESLint configurations where needed, then resolve the resulting violations; the migration is done when the targeted packages pass lint with the rule enabled.