[Feature Request]: sparse vector

### Is there an existing issue for the same feature request?

- [X] I have checked the existing issues.

### Is your feature request related to a problem?

```Markdown
Sparse vector. First proposal is to use pg_vector syntax.

SELECT '{0:1.5,2:3.5}/5'::sparsevec;
SELECT '{0:1.5,2:3.5}/5'::sparsevec::vector;
SELECT '{0:1.5,2:3.5}/5'::sparsevec::vector(5);
SELECT '{0:1.5,2:3.5}/5'::sparsevec::vector(4); -- this is an error, dim not match.
SELECT '[0,1.5,0,3.5,0]'::vector::sparsevec;
```

### Describe the feature you'd like

A new datatype sparse_vec.

### Describe implementation you've considered

_No response_

### Documentation, Adoption, Use Case, Migration Strategy

_No response_

### Additional information

_No response_