feat(cli): add autoloading of the config file

[BioPhoton]

Included in this PR is:

• split onlyPlugins middleware out of core config
• adjust execution of core config
• simplify core config
• adopt existing docs

closes #307

[!NOTICE]
Should wait until #358 and #363 is revolved

Update:
I moved in the second option as it cleans up the codebase without hacks.
Will open another PR for the config refactoring.

Proposed additional changes to this PR

Problem

ATM yargs is not configured correctly:

• NO configAPI is used
• options are without default values
• default handling is done in middleware

This setup is hard to setup, reason about maintain and extend as unintuitive and fragile.

Suggested solution

We suggest to use the config API of yargs and implement the default values correctly.

This would be like the following:

• configAPI is used and filled with potential existing code-pushup.config.(ts|mjs|js)
• options contain default values
• middleware gets removed completely

There are 2 possible ways to get close to the above solution:

Load RC config in a separate step before the CLI executes the command

As a proper solution is outside of this PR I suggest we discuss the following change:

1. Introduce the following function to the codebase of cli package

import {Argv, Options} from "yargs";
import {readCodePushupConfig} from "@code-pushup/core";

export async function withAsyncConfig<T>(unconfiguredCli: Argv<T>, optionsName = 'config', options: Options = {}) {
  // create a yargs CLI only configured to parse the config file
  const args = await unconfiguredCli.options({[optionsName]: options}).parseAsync();
  const configJson = await readCodePushupConfig(args[optionsName] as string);
  // configure cli with config
  return unconfiguredCli.config(configJson);
}

2. Make cli and yargsCli async and use the configure `yargs``

export async function yargsCli<T = unknown>(
  argv: string[],
  cfg: {
    // ...
    options?: { [key: string]: Options } & { config: Options };
  },
): Promise<Argv<T>> {
  // ...
  const cli = yargs(argv);

  const configuredCli = await withAsyncConfig(cli, 'config', options['config'])
  
  // ...
}

Execute the middleware before argument validation

A middleware can be added with the option {applyBeforeValidation: boolean}.
We can use it and at lease reduce the hack with default values a bit. (config middle ware solves multiple problems ATM)

Still not as clean as the above when it comes to reasoning etc...

[Tlacenka]

It is a great improvement overall, having the configuration extension not hardcoded.

The reason I put "request for changes" is the reduced coverage and missing documentation of the new logic.

[matejchalk]

Great job 👍 The tests look really good in the end 🙂 Really looking forward to using this feature and deleting --config code-pushup.config.ts everywhere 😁 🎉

[Tlacenka]

Overall a nice improvement!

Main feedback - the lost integration test coverage should be restored before merging.

[Tlacenka]

Test coverage restored.

Let us please not forget about the rest of the comments in a follow-up PR.