Add new GregClient constructor to accept custom http client

[aparajit-pratap]

Purpose

This adds a new constructor to initialize GregClient with a custom HTTP client to be used for no-network mode in Dynamo. See Dynamo PR DynamoDS/Dynamo#16238.

Declarations

Check these if you believe they are true

• The codebase is in a better state after this PR
• Is documented according to the standards
• The level of testing this PR includes is appropriate
• Changes to the API follow Semantic Versioning and are documented in the API Changes document.

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Pull Request Overview

This PR introduces a new GregClient constructor overload that accepts a custom HttpClient, enabling no-network mode in Dynamo and applying conditional TLS settings for older Windows versions.

• Imported System.Net.Http and added an overload to inject a custom HttpClient.
• Integrated optional HttpClient into RestClient initialization with duplicated TLS setup logic.
• Retained fallback to the existing RestClient constructor when no HttpClient is provided.

Comments suppressed due to low confidence (1)

src/GregClient/GregClient.cs:35

• The new constructor path with a custom HttpClient isn't covered by existing tests; consider adding unit tests to verify initialization in no-network mode.

public GregClient(IAuthProvider provider, string packageManagerUrl, HttpClient httpClient)