Source Productboard: users stream schema misses email and externalId

## What
The users stream in source-productboard currently exposes only companyId, id, and name.

## Problem
According to Productboard API docs for GET /users, responses can also include email and externalId. Missing these fields in the connector schema prevents users from selecting/syncing these columns.

Reference: https://developer.productboard.com/reference/getusers

## Expected
Update the users stream schema to include:
- email (nullable string)
- externalId (nullable string)

## Scope
Connector: airbyte-integrations/connectors/source-productboard
Type: backwards-compatible connector schema enhancement

## Notes
A patch is already prepared locally with:
- schema update in manifest
- connector version bump
- changelog entry

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**Internal Tracking:** https://github.com/airbytehq/oncall/issues/11506