Support directly inserting pgtype.Bits into a column of type BIT(n) using named args

**Is your feature request related to a problem? Please describe.**

Right now if i try and directly insert pgtype.Bits into column of type BIT(n) using named args i get an incorrect binary type error. as a solution im converting my pgtype.Bits into strings of "1"s and "0"s like "1010011...1111001"

**Describe the solution you'd like**

Ideally pgtype.Bits -> Bit(n) is supported. scanning from a query result into a pgtype.bits works funnily enough (BIT(n) -> pgtype.Bits)

Start by locating the pgtype.Bits encoding path and the named-argument parameter conversion for PostgreSQL BIT(n), then compare them with the existing BIT(n)-to-pgtype.Bits scan path. Done means a pgtype.Bits value can be inserted directly into a BIT(n) column through named arguments without an incorrect binary type error, with coverage for the conversion.

Written by the indexing model from the issue text.