feat: add support for config file

[tinchoz49]

Hey guys.

This PR adds support to specify a config file. So the user can have more control about the configuration.

For example: If I want to define all my dependencies as external I have to use a esbuild plugin or define in the serverless.yml the entire list of dependencies.

Using a config file lets the user to solve this by:

module.exports = () => ({
  external: Object.keys(require('./package.json').dependencies)
})

Without having to install new dependencies or esbuild plugins that can impact in the performance.

[floydspace]

@tinchoz49 it looks good, thanks for your contribution.
The extra point here: you could also use https://github.com/serverless/typescript to define serverless config as TS file and do the same thing with importing package.json dependencies. However, it still requires having 181-184 lines from your PR. so I give a green light. 👍

[tinchoz49]

Thank you @floydspace !

[github-actions]

🎉 This PR is included in version 1.31.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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