feat: external session

[Joozty]

Description

This PR adds support for sharing a single RUM session between mobile and browser agents by transferring session metadata across app boundaries.

• Added support for initializing Splunk Web RUM with externally provided session data via sessionMetadata in SplunkRum.init(...).
• Added SplunkRum.getSessionMetadata() to expose current session metadata for transfer to another agent.

Why

Today, flows that move users between native and browser contexts can create separate sessions (for example, MRUM in mobile and BRUM in browser).
With this change, session metadata can be passed from one agent to another so both sides continue the same session.

Example Scenario

A mobile app opens the system browser (not a webview) for login:

• Before: one mobile session + one browser session.
• After: pass session metadata from mobile to browser (for example via URL/deep link payload), initialize Web RUM with it, and continue a single shared session.

Type of change

Delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How has this been tested?

Delete options that are not relevant.

• Manual testing
• Added unit tests
• Added integration tests