Introduce PrimaryKeyConstraint

[morozov]

Currently, the primary key constraint is represented in DBAL as an index with the "primary" flag. This leads to various sorts of issues, for example:

1. It is possible to declare a primary key constraint with the properties of an index that don't make sense for a primary key (e.g. declare it as partial, full-text, etc.), which is invalid and will fail at runtime.
2. It is very challenging to add the support for primary constraint name (Use the provided index name for primary keys #3890).
3. Because of that, dropping a primary key constraint is not implemented on a half of the supported database platforms.
4. Table#renameIndex() likely doesn't work for primary key constraints.

This PR deprecates indexes with the "primary" flag in favor of PrimaryKeyConstraint.

Technical details

1. The setPrimaryKey() method is being deprecated in favor of Table::addPrimaryKeyConstraint(). The "add" verb better corresponds to the SQL syntax (ALTER TABLE <name> ADD CONSTRAINT). The currently existing "set" method already has the "add" semantics (a PK cannot be set twice).
2. The dropPrimaryKey() is not being deprecated. Even though there's now a slight inconsistency in naming, eventually Table will become immutable, so it will be deprecated. This way, I'm reducing the number of deprecations.
3. Similar to Introduce equals() for names and foreign key constraints #6866, PrimaryKeyConstraint#equals() will be added in 5.0.x, not here, since it's not necessary for the upgrade and isn't always possible to use.