[feature] Add support for captive-portal API

[nemesifier]

⚠️ Warning: not suited for beginner contributors. Requires more discussion.

Modern captive portals have RFC 8908 Captive Portal API.

A few useful ways we could use this API right now:

• Check if captive portal login is required.
• Check if we are in "internet-mode".

Constraints:

• The timeout should be short, eg: 2 seconds, but configurable, to avoid waiting for a long time in case something goes wrong.
• It shouldn't be required.
• The URL should be configurable per org.
• It should be documented.

[saurabh24thakur]

@nemesifier I have raise PR for that issue.

[nemesifier]

@saurabh24thakur your PR is closed.