feat(libstore/s3): add toHttpsUrl

[lovesegfault]

Motivation

This is extracted from the work in #13752

c.f. #13752 (comment)

Context

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Pull Request Overview

This PR adds a new toHttpsUrl() method to the ParsedS3URL class that converts S3 URLs to HTTPS URLs for use with curl's AWS SigV4 authentication. This enhancement supports better integration with HTTP-based S3 operations.

• Adds toHttpsUrl() method to convert S3 URLs to HTTPS format
• Handles various endpoint configurations including custom endpoints and AWS regions
• Includes comprehensive test coverage for different S3 URL conversion scenarios

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
src/libstore/include/nix/store/s3.hh	Adds method declaration and documentation for toHttpsUrl()
src/libstore/s3.cc	Implements the toHttpsUrl() method with endpoint handling logic
src/libstore-tests/s3.cc	Adds parameterized tests covering various S3 URL conversion scenarios

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