[10.x] Support check constraints

[hafezdivandari]

Laravel DB migrations supports almost all column types, indexes and constraints, except CHECK constraint. This PR adds support for check constraint on tables and columns without a breaking change.

Database	Supported version on Laravel	Check constraint support	Drop constraint support
MariaDB	10.3+	✓ (Docs)	✓ (Docs)
MySQL	5.7+	8.0.16+ (Docs)	8.0.19+ (Docs)
PostgreSQL	10.0+	✓ (Docs)	✓ (Docs)
SQLite	3.8.8+	✓ (Docs)	✗ (Docs)
SQL Server	2017+	✓ (Docs)	✓ (Docs)

Note: MySQL 5.7 premier support has ended on Oct 2020 and its extended supports ends on Oct 2023.

Examples:

• Add check constraints when creating table:

  Schema::create('users', function (Blueprint $table) {
      $table->integer('c1')->check('c1 > 10'); // `c1` int not null check (c1 > 10)
      $table->integer('c2');                   // `c2` int not null
      $table->check('c1 + c2 < 100');          // check (c1 + c2 < 100)
      $table->check('c2 <> 0', 'c2_nonzero');  // constraint `c2_nonzero` check (c2 <> 0)
  });

• Add check constraints when updating table (SQLite doesn't support this):

  Schema::table('users', function (Blueprint $table) {
      $table->check('c1 + c2 < 100');         // add check (c1 + c2 < 100)
      $table->check('c2 <> 0', 'c2_nonzero'); // add constraint `c2_nonzero` check (c2 <> 0)
  });

• Add a column with check constraint:

  Schema::table('users', function (Blueprint $table) {
      $table->integer('c2')->check('c2 <> 0'); // add `c2` int not null check (c2 <> 0)
  });

• Modify a column with check constraint (MySQL Only):

  Schema::table('users', function (Blueprint $table) {
      $table->integer('c2')->check('c2 <> 0')->change(); // modify `c2` int not null check (c2 <> 0)
  });

• Drop a constraint (SQLite doesn't support this):

  Schema::table('users', function (Blueprint $table) {
      $table->dropCheck('c2_nonzero')      // drop constraint `c2_nonzero`
      // Or alternatively:
      $table->dropConstraint('c2_nonzero') // drop constraint `c2_nonzero`
  });

[taylorotwell]

Thanks for your pull request to Laravel!

Unfortunately, I'm going to delay merging this code for now. To preserve our ability to adequately maintain the framework, we need to be very careful regarding the amount of code we include.

If possible, please consider releasing your code as a package so that the community can still take advantage of your contributions!

If you feel absolutely certain that this code corrects a bug in the framework, please "@" mention me in a follow-up comment with further explanation so that GitHub will send me a notification of your response.

[lorenzolosa]

@hafezdivandari By any chance, have you put this feature on a package or something like that? I'm currently using raw statements to manage check constraints, and it would be nice to have a better integrated solution.

[hafezdivandari]

@lorenzolosa Both Grammar and Blueprint classes are Macroable, so it's easy to extend them and add methods of this PR to each class on your application layer. I didn't see much of a demand from the community on this to release it as a package, maybe reconsidered later.