perf: optimization for async index build

[SNC123]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

#6756 (Tracking Issue)

Note: This PR will need to be rebased after the PR below are merged.

#7029 (Dependency, build types implement)

What's changed and what's your intention?

Implemented three optimization for async index build:

• Full SST Indexing Optimization: Builds indexes for entire SST files at once, replacing the previous incremental method.
• Task Resource Management: Limits resource usage of background tasks to maintain system performance.
• Task Priority Mechanism: Prioritizes background tasks, allowing critical operations to execute sooner.

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.