Add Gemini Resource

[shimon-cherrypick]

Summary & Motivation

• This PR adds support for the GeminiResource. In addition to wrapping the Gemini client ("GenerativeModel"), the resource will also automatically log usage-metadata (calls and tokens) to the AssetMetadata.

How I Tested These Changes

• Created mock-tests - testing both the get_model functionality, and the wrapping of the Gemini.generate_content call for metadata-logging (following as closely as possible the example of dagster-openai tests).
• Tested the resource against the real Gemini API, making sure that the usage stats are logged to the AssetMetadata.

Changelog

Ensure that an entry has been created in CHANGELOG.md outlining additions, deletions, and/or modifications.

See: keepachangelog.com