[Auth]: Auth provider "Sliplane" is not supported in ivy auth add CLI command

[ArtemLazarchuk]

Description

When running the ivy auth add command and selecting Sliplane as the auth provider, the CLI accepts the input (Client ID and Secret), but then fails with an error saying the provider is not supported.

Steps to reproduce

1. Run ivy auth add
2. Select Sliplane as the auth provider
3. Enter Client ID and Client Secret

Actual result

Auth Provider: Sliplane
Error: Auth provider 'Sliplane' is not supported.

Issue:

#1484

[nielsbosma]

Don't do this - Im thinking that we are to remove many of these CLI commands and just support them "agentically"