.Net: Add CachedContent Property to GeminiPromptExecutionSettings - #10268

…crosoft#10268)

### Motivation and Context

**Why is this change required?**  
This change introduces a new `CachedContent` field to the
`GeminiPromptExecutionSettings` class. The addition enables context
caching, a feature that optimizes the handling of repeated static
content in requests with high input token counts.

**What problem does it solve?**  
Repeatedly processing substantial, static context data (e.g., lengthy
documents, audio files, or video files) in requests can be
resource-intensive and costly. Context caching addresses this by
allowing the reuse of shared context across multiple requests, improving
performance and cost-efficiency.

**What scenario does it contribute to?**  
Context caching is particularly well-suited for scenarios where a
substantial initial context is repeatedly referenced by shorter
requests. Use cases include:
- **Chatbots with extensive system instructions**: Allows reuse of
complex system configurations across multiple user interactions.
- **Repetitive analysis of lengthy video files**: Enables efficient
analysis by caching video-related metadata or transcriptions.
- **Recurring queries against large document sets**: Improves efficiency
for workflows requiring repeated access to large document collections.
- **Frequent code repository analysis or bug fixing**: Facilitates the
reuse of large codebases as cached context for debugging or analysis
tasks.

*No open issues are linked to this change.*

---

### Description

This PR adds a `CachedContent` field to the
`GeminiPromptExecutionSettings` class. The field allows users to
reference cached context items such as text blocks, audio files, or
video files in prompt requests.

**Key features:**  
- The minimum size of a context cache is 32,768 tokens.  
- Cached content is retained for a default duration of 60 minutes, with
the option to configure expiration.
- Cached content is billed efficiently: the initial creation call is
charged at the standard rate, while subsequent references to the cache
are billed at a reduced rate.

By enabling context caching, users can optimize cost and resource usage
for workflows requiring substantial and repeat context data.

---

### Contribution Checklist

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

---

### Demo

<img width="1465" alt="Screenshot 2025-01-23 at 11 46 17 PM"
src="https://github.com/user-attachments/assets/9d070c69-a482-4a40-a912-2b17ddf4c7ab"
/>
<img width="841" alt="Screenshot 2025-01-23 at 11 50 29 PM"
src="https://github.com/user-attachments/assets/aac961ec-edf8-4458-a815-57e46f2a5789"
/>


---

### Notes

1. The CachedContent is only available via `v1beta1` endpoint
2. [Overview of context caching in
Gemini](https://cloud.google.com/vertex-ai/generative-ai/docs/context-cache/context-cache-overview?hl=en)
3. [cachedContent field in the
body](https://cloud.google.com/vertex-ai/generative-ai/docs/reference/rest/v1beta1/projects.locations.endpoints/generateContent)
for the `v1beta1` endpoint

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Co-authored-by: Roger Barreto <[email protected]>