The changes introduce a `Replacement` member to the vectorstores
options. This option can be used in `AddDocuments` function.
When Replacement is set to `true`, AddDocuments function will
delete old record with the same Metadata attribute. (i.e to cover
or to replace).
When Replacement is set to `false`(default case), AddDocuments
function will simply add documents without checking existing
document records.

- add doc for `WithReplacement` opt
- add test in pgvector_test