Add override hook PGDialect.set_backslash_escapes()

Add override hook PGDialect.set_backslash_escapes()

### Description
Refactor out the lines in `PGDialect.initialize()` that set backslash escapes into their own method to provide an override hook for [`sqlalchemy-redshift`](https://github.com/sqlalchemy-redshift/sqlalchemy-redshift) to use.

Fixes #9442

### Checklist

This pull request is:

- [ ] A documentation / typographical error fix
	- Good to go, no issue or tests are needed
- [x] A short code fix
	- please include the issue number, and create an issue if none exists, which
	  must include a complete example of the issue.  one line code fixes without an
	  issue and demonstration will not be accepted.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.   one line code fixes without tests will not be accepted.
- [ ] A new feature implementation
	- please include the issue number, and create an issue if none exists, which must
	  include a complete example of how the feature would look.
	- Please include: `Fixes: #<issue number>` in the commit message
	- please include tests.

Closes: #9475
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/9475
Pull-request-sha: 5565afeac20ea3612c3f427f58efacd8487ac159

Change-Id: I9b652044243ab231c19ab55ebc8ee24534365d61