Migrate to Auth Identity Language Server plugin instead of Device SSO command

## Description

Currently plugin uses [Device SSO Auth command Language Server extension](https://github.com/aws/language-servers/tree/main/server/device-sso-auth-lsp) to manage SSO authentication and caching of token on filesystem. This server extension is experimental and temporary.

Migrate plugin [login flow](https://github.com/awslabs/amazonq.nvim/blob/main/lua/amazonq/sso.lua#L30-L63) command to use new standard [aws-lsp-identity: Identity server](https://github.com/aws/language-servers/tree/main/server/aws-lsp-identity). That server implements set of LSP protocol extensions to request SSO token. It also manages SSO authentication flow and management of token on disk in robust way.

* Identity Management LSP protocol extension, implemented in Identiyy Server: https://github.com/aws/language-server-runtimes/tree/main/runtimes#identity-management
* https://github.com/aws/amazon-q-eclipse/pull/125 for reference integration with Identity Server.

Start in lua/amazonq/sso.lua at the login flow on lines 30-63, then read the Identity Management LSP protocol and the aws-lsp-identity server. The Eclipse integration in amazon-q-eclipse#125 is the stated reference. Done means the plugin uses the standard Identity server instead of the experimental Device SSO Auth server for authentication and token management.

Written by the indexing model from the issue text.