feat: `SignIn` component for `@asgardeo/nextjs`

[brionmario]

Is your feature request related to a problem? Please describe.
We need a built-in SignIn component in the @asgardeo/nextjs SDK to make it easier for developers to quickly embed a login UI without having to build it from scratch. Currently, developers must manually call signIn() and handle redirections, which increases integration complexity.

Describe the solution you would prefer
A reusable and customizable SignIn React component that:

• Internally triggers the signIn() flow from the SDK.
• Accepts props for UI customization (labels, styles, icons).
• Supports callback hooks for success and failure.
• Plays nicely with SSR environments like Next.js.
• Leverages the i18n system for multi-language support.
• Supports different login strategies (e.g., redirect, popup, app-native).

The implementation should align with the App Native Authentication guidelines documented here:
👉 https://is.docs.wso2.com/en/7.1.0/references/app-native-authentication/

Additional context
This component will be useful for:

• Rapid prototyping and onboarding.
• Avoiding repeated implementations across apps.
• Ensuring consistency in login UX across all apps using @asgardeo/nextjs.

We should consider extending this later to include a combined AuthForm with support for sign-up and federated logins.``