Make portal-client dependency optional

[matejchalk]

User story

In order to streamline approval process in enterprises which vet every NPM package, it helps if the Code PushUp CLI has as few external dependencies as possible.

The @code-pushup/portal-client (which also pulls in graphql, graphql-request and graphql-tag) is only needed for uploading to portal. That means its installation could be optional.

Acceptance criteria

• [ ] change @code-pushup/portal-client imports from static to dynamic
• [ ] move @code-pushup/portal-client from dependencies to optionalDependencies
• [ ] add additional portal setup instructions to documentation