.Net: Allow custom serialization options to be passed to TextMemoryPlugin.

[westey-m]

Motivation and Context

When TextMemoryPlugin gets back multiple results from the memory store during recall, it uses json serialization to
turn these multiple results into a single string.
In some cases, this may cause unintended side-affects, e.g. when non-latin script is present in the results, JsonSerializer automatically escapes these characters.
Adding support for passing custom JsonSerializerOptions into the TextMemoryPlugin to customize the serialization behavior.

See #6593

Description

Added optional JsonSerializerOptions to the TextMemoryPlugin constructor.
Added example to demonstrate this scenario.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄