Add registration for metadata of table

[ragi256]

Related to #292

There are many other systems around Redshift as a DWH DB.
For example, data pipeline jobs, streaming load, ETL batch.
It would be useful to be able to register various metadata of a table to Dmemo from other systems.

(And if possible, I want to add API to provide metadata.)

If this feature is added, Dmemo can also be a convenient metadata repository, and there will be more ways to use it.