Support custom iframe attributes for silent signin

[Sune1337]

This PR adds support for passing custom HTML attributes to the iframe used during silent signin (signinSilent).

Motivation
Recent Chrome changes around Local Network Access (PNA / Private Network Access) require explicit permission delegation for iframe-based requests to local or private network resources.

A common real-world scenario is:

A web application hosted on a private/internal address (e.g. https://app.local or an internal DNS/IP) using a public identity provider such as Microsoft Entra ID (Azure AD).

In this setup:

• The main application runs on a private/local network
• Authentication happens via a public endpoint (e.g. login.microsoftonline.com)
• Silent signin is performed using a hidden iframe

With the latest browser restrictions, this iframe may be blocked unless it explicitly declares permission via:

<iframe allow="local-network-access *">

Without this attribute:

• Silent authentication can fail
• Users may be unexpectedly logged out or forced into full redirects

Changes
Adds optional iframeAttributes configuration:

• Available globally via UserManager settings
• Available per call via signinSilent arguments

Example usage
Global configuration

const userManager = new UserManager({  iframeAttributes: {    allow: "local-network-access *"  }});

Per-call override

await userManager.signinSilent({  iframeAttributes: {    allow: "local-network-access *"  }});

Notes

• Using the feature is optional and should not change any behavior if not used.
• Supports arbitrary attributes, allowing future browser requirements to be handled without further changes

[pamapa]

looks good, thanks for contributing