db: add expired_at and last_valid_at columns to user_external_accounts table

Created by: unknwon

Adds two columns to the user_external_accounts table:

• expired_at indicates when the token stored in the auth_data has been detected expired.
• last_valid_at indicates when was the last successfully used, later will be used for workerutil package (@ryanslade could you confirm the workerutil package requires naming convetions of the column or it is OK to customize the column name for workers to pick up?).

This two columns are not yet used but will be when addressing both https://github.com/sourcegraph/sourcegraph/issues/14899 and https://github.com/sourcegraph/sourcegraph/issues/15176.