feat: add *sql.DB connector that uses database context

[lzakharov]

• Do only one thing
• Non breaking API changes
• Tested (looks unnecessary)

What did this pull request do?

This PR adds ability to provide *sql.DB connector which can resolve current connect based on the database context. It works the same as GetDBConnector but additionally have *gorm.DB state.

User Case Description

Since we use DBResolver plugin to provide multiple databases support, sometimes it's necessary to get raw *sql.DB. At this moment all these statements return the same connection pool, ignoring provided clauses:

db.DB()
db.Clauses(dbresolver.Write).DB()
db.Clauses(dbresolver.Read).DB()
db.Clauses(dbresolver.Use("slow")).DB()

However, it seems to us more logical behavior to return different connections based on the current database state.