.Net: MEVD: Add embedding generation for AzureAISearch

[westey-m]

Currently we don't have built-in embedding generation for AzureAISearch because:

1. Embedding generation can also happen in the service and we need to determine a fallback strategy
2. There is currently no location to insert the generated vectors before upsert for POCO objects since serialization is done by the SDK.

Suggesting that we take over serialization for POCO scenarios and use the locally configured embedding generator if available and fall back to the service only if not provided.