Allow passing custom environment variables to FFmpeg

[Tyrrrz]

using YoutubeExplode;
using YoutubeExplode.Converter;

var youtube = new YoutubeClient();
var videoUrl = "https://youtube.com/watch?v=u_yIGGhubZs";

await youtube.Videos.DownloadAsync(videoUrl, "video.mp4", o => o
    // Set custom variables
    .SetEnvironmentVariable("FFREPORT", "file=foo.log")
);

[Copilot]

Pull Request Overview

This PR introduces the ability to pass custom environment variables to FFmpeg.

• Updated FFmpeg to accept and utilize a dictionary of environment variables.
• Modified ConversionRequestBuilder and ConversionRequest to include environment variables configuration.
• Added tests to validate the custom environment variables functionality.

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
YoutubeExplode.Tests/StreamSpecs.cs	Removed an unused using directive for better code clarity.
YoutubeExplode.Converter/FFmpeg.cs	Updated the FFmpeg constructor and execution call to support environment variables.
YoutubeExplode.Converter/ConversionRequestBuilder.cs	Added a SetEnvironmentVariable method and introduced a dictionary for environment variables.
YoutubeExplode.Converter/ConversionRequest.cs	Modified constructors to accept and expose environment variables.
YoutubeExplode.Converter/ConversionExtensions.cs	Updated instantiation of FFmpeg to forward environment variables.
YoutubeExplode.Converter.Tests/GeneralSpecs.cs	Renamed an integration test for improved clarity.
YoutubeExplode.Converter.Tests/EnvironmentSpecs.cs	Added new tests to ensure proper handling of custom environment variables.