Comments on (named) constraints

Comments on (named) constraints

Adds support for comments on named constraints, including `ForeignKeyConstraint`, `PrimaryKeyConstraint`, `CheckConstraint`, `UniqueConstraint`, solving the [Issue 5667](https://github.com/sqlalchemy/sqlalchemy/issues/5667).

Supports only PostgreSQL backend.

### Description

Following the example of [Issue 1546](https://github.com/sqlalchemy/sqlalchemy/issues/1546), supports comments on constraints. Specifically, enables comments on _named_ ones — as I get it, PostgreSQL prohibits comments on unnamed constraints.

Enables setting the comments for named constraints like this:
```
Table(
   'example', metadata,
   Column('id', Integer),
   Column('data', sa.String(30)),
   PrimaryKeyConstraint(
       "id", name="id_pk", comment="id_pk comment"
    ),
   CheckConstraint('id < 100', name="cc1", comment="Id value can't exceed 100"),
   UniqueConstraint(['data'], name="uc1", comment="Must have unique data field"),
)
```

Provides the DDL representation for constraint comments and routines to create and drop them. Class `.Inspector` reflects constraint comments via methods like `get_check_constraints` .
### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
- [ ] A short code fix
- [x] A new feature implementation
	- Solves the issue 5667.
	- The commit message includes `Fixes: 5667`.
	- Includes tests based on comment reflection.

**Have a nice day!**

Fixes: #5667
Closes: #7742
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7742
Pull-request-sha: 42a5d3c3e9ccf9a9d5397fd007aeab0854f66130

Change-Id: Ia60f578595afdbd6089541c9a00e37997ef78ad3