FEAT: Target Registry and AIRT Targets Initializer

[jsong468]

Description

This PR adds a Target Registry feature for storing instances of targets to be re-used and an AIRT initializer that instantiates a wide range of commonly used targets using environment variables. These additions will be useful for the GUI as they enable us to load commonly used targets and have them ready to use.

• TargetRegistry follows the pattern of the existing ScorerRegistry
• The AIRTTargetInitializer is a PyRITInitializer subclass that scans environment variables and registers available targets into the TargetRegistry. No environment variables are strictly required. The supported targets include OpenAIChatTarget, OpenAIResponseTarget, RealtimeTarget, OpenAIImageTarget, OpenAITTSTarget, OpenAIVideoTarget, and PromptShieldTarget

Tests and Documentation

Unit tests for TargetRegistry added

TODO

• Unit tests for AIRTTargetInitializer
• Maybe notebook example in 2_instance_registry