feat: support parallel table operations in COPY DATABASE

[WenyXu]

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What's changed and what's your intention?

This PR introduces parallelism improvements for both COPY DATABASE operations and CLI export functionality to enhance performance when handling multiple tables.

Changes

1. Support parallel table operations in COPY DATABASE

• Added parallelism option to COPY DATABASE operations (defaults to total CPU cores)
• Tables are now processed in parallel instead of sequentially
• Enhanced logging to show progress with table count (e.g., "Copy table(1/10): ...")
• Added common-stat dependency to get system CPU core count

2. Add parallelism parameter to CLI export

• Added new --parallelism flag to export command (default: 8)
• Renamed internal parallelism field to export_jobs for clarity
• The parallelism parameter is now passed to SQL queries via WITH clause

Example Usage

COPY DATABASE with custom parallelism

COPY DATABASE my_db TO 'file:///path/to/backup/' WITH (format='parquet', parallelism=16);

CLI export with both parameters

greptime cli export --database my_db --export-jobs 4 --parallelism 16

PR Checklist

Please convert it to a draft if some of the following conditions are not met.

• I have written the necessary rustdoc comments.
• I have added the necessary unit tests and integration tests.
• This PR requires documentation updates.
• API changes are backward compatible.
• Schema or data changes are backward compatible.

[killme2008]

LGTM

[fengjiachun]

Rest LGTM