"optional" properties for expression.EvalContext

[lcwangchao]

The EvalContext has many fields to provide but not all of them are required in every scene. So it's better to classify these fields into two categories:

1. Basic properties such as type.Context, errctx.Context, SQLMode, etc... These properties are used by functions widely. For example, errctx.Context used by most functions to handle some errors. The basic properties are also simple, they should be easy to clone and mock. The EvalContext MUST provide basic properties.
2. Optional properties, as its name, is "optional". In some scenes such as lightning, it only requires some of the properties, so these properties are provided on demand. Optional properties are not widely used, for example, tidb_is_ddl_owner requires EvalContext to provide the information on whether the current tidb is DDL owner, but this function is forbidden in most other scenes except SQL query, like partition expressions or generated column expressions. Some optional properties are complex, so not all of them can be cloned or migrated.