Add ability to define external schema names in Redshift datashare requests

[dlpzx]

Is your idea related to a problem? Please describe.
This is an enhancement request for the Redshift data sharing feature to be released in 2.7.0 #955

Redshift specialists have identified improvements on the current implementation and next steps. One of which is the ability for users to define the external_schema name. This is especially relevant for the case of Redshift migrations from cluster to cluster. If the users have many queries referencing an schema name it will be very cumbersome for them if they cannot control the changes on the schema names.

Describe the solution you'd like
The data requesters in a data.all redshift request should have the ability to introduce the name of the external schema they would like to create