Add drop all option & bug fix

[Matan Ben-Tov (matanbt)]

Hi!

First, thanks for maintaining this useful wrapper for Milvus :) Using it, I encountered a small issue, and a feature that I had to implement externally; so I thought I'd add them here.

• search_by_metadata(): The function assumes the text field is requested by the function caller, although they might require metadata alone. Such requests currently result in an exception. Thus, to ensure text field is always returned on the database query, I suggest simply adding it, when missing.

• drop(): Resetting the vectorstore by dropping all rows is very useful, and in fact currently enabled only on initialization (as part of __init__()). I suggest to decouple it into a method (drop(); feel free to offer other names), exposing it for external use.

[Cheney Zhang (zc277584121)]

Matan Ben-Tov (@matanbt) thanks, it's a good optimization, but some lint check failed,please fix it according to the CONTRIBUTING.md guide. And btw, the async function asearch_by_metadata is the same structure as the sync one , can you change them in the same way

[Matan Ben-Tov (matanbt)]

• Fixed linting (now pass the local lint checks).
• Added the required change to asearch_by_metadata.