.Net: [MEVD] Consider getting the dimensions from the configured IEmbeddingGenerator

[roji]

IEmbeddingGenerators optionally provide metadata which can include DefaultModelDimensions (code). MEVD connectors could read that value (when available) and use that as the dimensions setting for vector properties. This would mean that we make dimensions optional on user-facing APIs for configuring vector properties (record definition, attribute).

Since this is an optional value not exposed by all embedding generators (and since users don't have to configure an embedding generator at all), we'd throw at validation-time if it's missing.

/cc @westey-m @adamsitnik