Adding new methods and models support

[gbaptista]

• Detaching model from base URL;
• Adding the new predict and embed_content methods;
• Validating support for Gemini 1.5 Pro and Gemini 1.5 Flash;
• Adding documentation for Embeddings generation;

Breaking Change: client.request now requires a longer portion of the path:

Before:

client.request(
  'streamGenerateContent',
  # ...
)

After:

client.request(
  'publishers/google/models/gemini-pro:streamGenerateContent',
  # ...
)

This breaking change should not be impactful (we infer that most users don't use client.request directly) and will enable the Gem to evolve in the future to support more methods and requests that are not necessarily attached to a specific model.