Basic implementation and basic tests added for using sparsevec introduced in pgvector 0.7.0.

[cho-thinkfree-com]

I have written the basic code to use sparsevec added in pgvector 0.7.0. I have confirmed that basic operations are functioning correctly.

Only the part handling Query as String has been implemented, and the part using binary has not been included as I am not aware of how it operates.