lib/logstorage: add coalesce pipe, update documentation

[warkadiusz]

Describe Your Changes

Implements the coalesce pipe for LogsQL, which returns the first non-empty value from a list of fields.

• Added coalesce implementation and tests
• Updated LogsQL documentation and examples

Fixes #690

Example usage:

• Return new field

  * | coalesce (user_id, username, email) as user 

• Return new field with default:

  * | coalesce (user_id, username, email) as user default "anonymous" 

• Overwrite existing field with the first non-empty value:

  * | coalesce (user_id, username, email) as user_id 

Looking forward to your feedback and happy to make any changes needed.

Checklist

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• My change adheres to VictoriaMetrics contributing guidelines.
• My change adheres to VictoriaMetrics development goals.