feat: add backend field to fulltext options

[zhongzc]

I hereby agree to the terms of the GreptimeDB CLA.

Refer to a related PR or issue link (optional)

Wait for GreptimeTeam/greptime-proto#226

What's changed and what's your intention?

Add a backend field to FulltextOptions to prepare for future multi-backend support in fulltext index. Currently supports:

• Bloom
• Tantivy

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.

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[fengjiachun]

LGTM

[fengjiachun]

@zhongzc CI failed

[killme2008]

Good job! I wonder what happens in the following scenario:

• A column uses the tantivy backend initially, some rows are inserted, and indexes are built.
• The backend is altered to bloom.

Questions:

• Do the old tantivy indexes still work?
• Can both the new and old indexes work together?

[zhongzc]

The application of the index is determined by the generated index file, not the table structure, so the original index file can still take effect.

The problem translates to whether both bloom and tantivy can work for the same query.

In principle, whether it's matches or matches_term, both have the potential to leverage bloom and tantivy for filtering and query acceleration. However, in the current implementation, matches_term is bound to bloom while matches is bound to tantivy. Thus, when using matches, the tantivy backend index takes effect, and when using matches_term, the bloom backend index takes effect. In the future, there may be opportunities to further refine and interconnect their applications.