.Net: Fix #6030 - Mitigating Prompt Injection in Liquid Templates (#6048

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### Motivation and Context

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#6030 
### Description
~In this implementation, the `Ġ` will be reserved in liquid template
which is used to replace `:` in all input variables when unsafe content
is not allowed.~

~The encoding process for input variables when unsafe content is not
allowed is~
~- replace all `:` to `Ġ` // this is the extra step comparing with
HandlerBar Template~
~- Encode xml using `HttpUtility.HtmlEncode`~

~The decoding process is~
~- replace all `Ġ` to `:`~


This PR introduces a new process to mitigate potential prompt injection
attacks from input variables when using liquid templates. Here's a
breakdown of the steps:

Before rendering, each input variable undergoes a transformation: all
occurrences of `:`are replaced with `&#58;`. This ensures that message
tags like `system:`, `user:`, or `assistant:` are not present if
`AllowUnsafeContent` is set to `false`. No replacement occurs if
`AllowUnsafeContent` is `true`.
After rendering, each message content is processed based on the
`AllowUnsafeContent` setting. If it's `false`, all `&#58;` instances are
reverted back to `:`, followed by calling `html_encode` on each message
content. If `AllowUnsafeContent` is `true`, only `html_encode` is
called. This additional encoding step is necessary because
`ChatPromptParser` always decodes XML message content, requiring the
liquid template to undergo an extra encoding step to ensure the rendered
content matches the original before rendering.

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### Contribution Checklist

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raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄