NL Model usage update: 1)custom DC dockerfile download model using gsutil; 2) model map object consolidation

[shifucun]

Instead of having a download mode in build_custom_dc_embeddings tool, download the model and embeddings directly in the docker image.

• Since we won't upgrade the sentence transformer based model, can just hardcode the only version in dockerfile
• Updated the base image to include gcloud

This is a step towards consolidating the build embeddings tool.

Part of this change consolidates the embedding and reranking model object in registry into one map.