[cmd/telemetrygen] Add span links support to telemetrygen

[IshwarKanse]

Description

This PR adds span links support to telemetrygen, enabling the generation of spans with links to previously created spans.
The new --span-links flag allows users to specify the number of span links to generate for each span, creating
relationships between spans for testing distributed tracing scenarios.

Key features:

• New --span-links command line flag to control the number of links per span
• Links are created to random existing span contexts from previously generated spans
• Each link includes attributes for link type and index identification
• Both parent and child spans can have links generated
• Thread-safe collection and access of span contexts for linking

Testing

• Added unit tests for span link generation functionality
• Tested with various --span-links values (0, 1, 5, 10)
• Verified links are properly attached to both parent and child spans
• Confirmed thread-safe operation with multiple workers
• Validated link attributes are correctly set

Documentation

• Updated command line help text for the new --span-links flag
• Added configuration documentation for the NumSpanLinks field

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[andreasgerstmayr]

LGTM, nice work 🚀!
I added two minor suggestions.

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