Add WithConnection to Postgres similar to MySQL.

[AndreasKl]

Enable consumers to provide an existing *sql.DB with the option to close the connection without closing the underlying *sql.DB instance.

See the following thread for reference and how it was implemented for MySQL:
#578
#583

[coveralls]

Pull Request Test Coverage Report for Build 1875336797

• 20 of 24 (83.33%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 57.854%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
database/postgres/postgres.go	20	24	83.33%

Totals
Change from base Build 1821224966:	0.05%
Covered Lines:	3742
Relevant Lines:	6468

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💛 - Coveralls

[AndreasKl]

@dhui Hi, can I improve the PR to get the change merged?

[c0olix]

+1 for this feature.

[dhui]

Thanks for the PR and your patience!