milvus: add skipFlushOnWrite option

[fatedier]

When importing a large number of documents at once, flushing during each AddDocuments can significantly increase the time taken.

In my scenario, I create a new collection, insert datas, and then replace the original collection, so there is no need to flush every time data is inserted.

Add an option skipFlushOnWrite to optimize this scenario. In my test scenario, the total time was reduced from 38 minutes to 5 minutes.

PR Checklist

• Read the Contributing documentation.
• Read the Code of conduct documentation.
• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. Fixes #123).
• Describes the source of new concepts.
• References existing implementations as appropriate.
• Contains test coverage for new functions.
• Passes all golangci-lint checks.

[tmc]

LGTM - consider linking to docs for this option.