FLIP-107: Expose metadata for new Table source

[crazyzhou]

Problem description
FLIP-107 has added the support for metadata along with the event for both table source and sink.

For table source, we can export the scope, stream, EventPointer in the table source, this can cover the benifit similar with #180 for DataStream Source.

e.g. the structure of the metadata

enum ReadableMetadata {
		SCOPE(
			"scope",
			DataTypes.STRING().notNull()
		),

		STREAM(
			"stream",
			DataTypes.STRING().notNull(),
		),

		EVENT_POINTER(
			"event-pointer",
			DataTypes.BYTES().notNull()
		)

Problem location
Table API

Suggestions for an improvement