Support for getting do_token from Vault

**What is the problem this feature would solve? Please describe.**
Now, `doctl` after auth init action, saves long-lived token for DO resources in a file system (config file).

**Describe the solution you'd like**
`doctl auth` should allow bypass the local storage and get the desired token from Vault repository. In that case, short-lived Vaults tokens will allow to getting long-lived. Similar features are available in terms of shadowing secrets on well-known cloud providers as with the AWS, Azure etc.

So the desired scenario might look like:
1. Call `doctl auth init` for getting Vault connection properties.
2. Call `vault login` in order to login into Vault (e.g. via OIDC provider, GitHub etc.)
3. Use `doctl` as usual, but in the background, `doctl` uses Vault client and honoured the default token from `~/.vault-token` in terms of getting target DO token.

**Additional context**
This suggestion improves significantly security in case of using `doctl` on the desktop env. It avoids storing DO tokens locally on fs.