feat: make portal-client dependency optional

[hanna-skryl]

Closes #474

• Make @code-pushup/portal-client an optional dependency in the @code-pushup/core package.
• Update the core package to use dynamic imports for @code-pushup/portal-client.
• Add a utility function to handle the loading of the portal client.
• Update the installation instructions to guide users on omitting optional dependencies, including details for npm, pnpm, and yarn setups.