Add custom actions with Gemini

[vsukhoml]

Use case
I'm working on refactoring existing code, and want to check if there is a better way to write certain things, fix potential security issues. Currently out of the box I get "Generate code", "Explain this", "Write unit tests" actions for a code selection. This is very limited and doesn't include many more useful applications where I have to use chat with copying code, etc.

Feature

1. Add out of the box actions like "Suggest how to improve code" or "Identify and suggest fixes for any potential security issues".
2. Make it possible to create a workspace & language specific custom actions where more context can be placed.
3. Add means to define workspace specific context - what this project is intended to do, execution environment, available libraries, frameworks, etc.