Introduce table editor

[morozov]

Q	A
Type	feature

Motivation

The introduction of the table editor should facilitate the following improvements:

1. Eventually make tables immutable.
2. Reduce the amount of validation that needs to be performed in the case of multi-step modification of a table (see Deprecate dropping columns referenced by constraints #6559 (comment)).
3. Get rid of this hack:

   dbal/src/Schema/Schema.php

   Lines 305 to 309 in e7e2615

   	$identifier = new Identifier($newName);
   	$table->_name = $identifier->_name;
   	$table->_namespace = $identifier->_namespace;
   	$table->_quoted = $identifier->_quoted;

   I just realized that replacing it with building a new table would be a BC break because the editor would create a new table instance instead of modifying an existing one.
4. Unify the process of table creation. For instance, currently, table columns can be specified in the constructor or added via addColumn() but the primary key can be set only via setPrimaryKey().

Being able to get rid of the hack is why I'm introducing the editor now. I want to start deprecating end removing the name-related properties of AbstractIdentifier in favor of the recently introduced value objects, but this hack won't with the new properties because they are private (not protected).

Implementation details

The editor (specifically, the class and method naming) is modeled after the table editor in Debezium.

I implemented the necessary minimum of the methods that would allow to rework the hack but then realized that it's a BC break. However, this API allows to replace the direct usage of the Table constructor in the library code with the builder, so I think we can introduce it as is.

I didn't touch the tests because

1. This is a lot of mechanical work (I can do it later).
2. The test code that uses the builder won't be as streamlined as the library code because the builder doesn't yet support all initial table properties (e.g. the primary key).

Additional changes

The first two commits make minor changes in the code that wasn't released yet.

[morozov]

This contract enables this expression:

dbal/src/Schema/Table.php

Line 867 in 7a25c5d

->setName($this->getObjectName()->toString())

[morozov]

Even though getObjectName() can throw NameIsNotInitialized, this isn't a BC break because Table::edit() is a new method not called from anywhere. It will be used to rename the table in 5.0.