Add the Content caching feature (#167)

## APIs added
- GoogleAICacheManager class (exported from new /server path) with create, update, get, list, delete methods.
- GoogleGenerativeAI.getGenerativeModelFromContextCache() added

## Infrastructure changed
- `/files` subpath changed to `/server`. GoogleAIFileManager and GoogleAICacheManager will be exported from here as they are only meant to be used in a server environment.
- All types moved to a single folder.
- Generate common/api-review md files for easier review of API mistakes/gaps
- Refactor `makeRequest`-related functions to reduce code duplication

.changeset/tough-shirts-cheat.md

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+---
+"@google/generative-ai": minor
+---
+
+Added `GoogleAICacheManager` utility to allow caching large content to be used in inference. This class is exported from the `@google/generative-ai/server` subpath. Breaking change: The `GoogleAIFileManager` class has been moved to be exported from this subpath as well instead of the `/files` subpath.