reduce duplication in `ConnectionAdapters::Table` docs. [ci skip]

Most of the documentation very closely mirrors the matching
docs from `SchemaStatements`. I reduced duplicated copy and
added links to the underlying methods for the user to follow.
add autoload for `ForeignKeyDefinition`.

/cc @sgrif
Support for any type primary key.
Add bigint primary key support for MySQL.

Example:

    create_table :foos, id: :bigint do |t|
    end
Merge pull request #18220 from kamipo/bigint_primary_key_support

Add bigint primary key support for MySQL.