IF NOT EXISTS should be handled inside DDL interface

[lance6716]

Feature Request

Is your feature request related to a problem? Please describe:

for CREATE DATABASE, IF NOT EXISTS is handled in DDL executor

tidb/executor/ddl.go

Lines 309 to 314 in 095e323

	err = domain.GetDomain(e.ctx).DDL().CreateSchema(e.ctx, model.NewCIStr(s.Name), opt, placementPolicyRef)
	if err != nil {
	if infoschema.ErrDatabaseExists.Equal(err) && s.IfNotExists {
	err = nil
	}
	}

for CREATE TABLE, it's handled in DDL interface

tidb/ddl/ddl_api.go

Lines 2171 to 2176 in 095e323

	onExist := OnExistError
	if s.IfNotExists {
	onExist = OnExistIgnore
	}
	return d.CreateTableWithInfo(ctx, schema.Name, tbInfo, onExist)

Describe the feature you'd like:

they should be unified, so another implementation of DDL interface can have a more reasonable behaviour.

I prefer we let DDL interface handle it.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: