[FR] Add support for aiembed with Google models

[svilupp]

It would be good to add the support for aiembed with GoogleGenAI, as the package + models support the functionality: https://tylerjthomas9.github.io/GoogleGenAI.jl/dev/#Create-Embeddings

It would require updating src/llm_google.jl with the aiembed method + updating the extension ext/GoogleGenAIPromptingToolsExt.jl to forward the underlying embed_content call.

The PR should:

• follow the similar patterns as the aigenerate
• have necessary docstrings with examples
• have corresponding tests