Add missing BigQuery connector configuration properties to documentation

[shunki-fujita]

Description

Several BigQuery connector configuration properties are defined in the code but missing from the documentation:

• bigquery.metadata.parallelism — Limits the parallelism of metadata enumeration calls (1–32, default: min(available CPUs, 32))
• bigquery.metadata-page-size — The number of metadata entries retrieved per API request (default: 1000)
• bigquery.job.label-name — Adds a label with the given name to the BigQuery job
• bigquery.job.label-format — Value format for the job label, supporting $QUERY_ID, $SOURCE, $USER, $TRACE_TOKEN
• bigquery.projection-pushdown-enabled — Enable dereference push down for ROW type (default: true)

[ebyhr]

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