[CT-2570] Instantiate Flags from dict

[stu-k]

Description

As part of the work for dbt retry #7299, we need to be able to instantiate a Flags class from a dict (with unserialized values) and name of a command alone. This will support the creation of Flags and RuntimeConfig for a command being retried.

Acceptance Criteria

• Flags can be instantiated provided only the following
  • the string name of a command
  • an unserialized dict representing the args used in a command