[UX]: Add dstack project CLI to configure, list and switching between projects

[peterschmidt85]

Fixes #2553

Benefits

Easily switch between local and hosted servers and view the current user configuration using the new dstack project command.

How it works

The new dstack project command replaces the existing dstack config command.

1. dstack project (same as dstack project list)

$ dstack project

 PROJECT         URL                    USER            DEFAULT
 peterschmidt85  https://sky.dstack.ai  peterschmidt85
 main            http://127.0.0.1:3000  admin              ✓

2. dstack project set-default

$ dstack project set-default peterschmidt85
OK

3 dstack project add (similar to old dstack config, but --project is changed to --name)

$ dstack project add --name peterschmidt85 --url https://sky.dstack.ai --token 76d8dd51-0470-74a7-24ed9ec18-fb7d341
OK

[r4victor]

Why leaving documentation for dstack config with an example for dstack project.

[r4victor]

I'd clarify that dstack project manages only the CLI configuration. Otherwise, users may get an impression that it can add and delete projects on the server.