[FEATURE] Define model used for embeddings (esp. also azure endpoints)

[danieldekay]

Describe the bug
Currently the embedding model is hard-coded in

storm/knowledge_storm/encoder.py

Line 83 in e80d9bb

self.embedding_model_name = "text-embedding-3-small"

This can be an issue in many ways:

• You want to use different model providers
• You don't have enough quota on text-embedding-3-small
• You use an Azure region where text-embedding-3-small is not available

Describe the feature

• Use an Environment Variable to specify the Embedding model, e.g. 'azure/text-embedding-ada-002' as well as the azure endpoint - which could be different to the completion model endpoints.