Add support for key length in MySQL

[stevefan1999-personal]

Motivation

In MySQL BLOB types cannot be primary key directly, and instead you must add a simple type constraint outside to make it a primary key indirectly, although it can still be conveniently done within CREATE TABLE like the following example:

create table kvstore
(
    `key` longblob not null,
    value longblob not null,
    constraint kvstore_pk
        primary key (`key`(3072))
);

Proposed Solutions

Using the aforementioned SQL DDL script, we should have a size constraint on primary_key which should correspond to:

#[derive(Clone, Debug, PartialEq, Eq, DeriveEntityModel)]
#[sea_orm(table_name = "kvstore")]
pub struct Model {
	#[sea_orm(
		primary_key(size = 3072),
		indexed,
		auto_increment = false,
		column_type = "Binary(BlobSize::Long)"
	)]
	pub key: Key,
	#[sea_orm(nullable = false, column_type = "Binary(BlobSize::Long)")]
	pub value: Val,
}

Additional Information

This issue is found by surrealdb/surrealdb#1717, which is trying to use MySQL as a backend database, or let's say all databases that SeaORM supports.