Add ability to generate PR description from diff in a file or url

[mkarle]

When integrating SKonsole into a GitHub Action, a problem arose when comparing changes from two different repositories. To get "git diff ..HEAD to work, the Action had to be in a specific state where the source branch had to be added as a downstream remote and then checked out. It was a headache, and I found that both Pull Request- and Issue comments-based actions have access to a diff url such as this one showing a README change.

This PR's diff is found here.
Result from dotnet run pr description --outputFormat json -d https://patch-diff.githubusercontent.com/raw/lemillermicrosoft/skonsole/pull/17.diff:

{
  "Title": "Add diff input file option to PRCommand",
  "Summary": "This pull request adds a new option to the PRCommand, allowing users to specify a diff input file or URL. This feature enhances the flexibility of the command by enabling users to provide custom diff content for generating pull request descriptions.",
  "Changes": [
    "Added a new option `--diffInputFile` (alias `-d`) to PRCommand, allowing users to specify a diff input file or URL",
    "Modified the `GeneratePRDescriptionCommand` method to include the new `diffInputFileOption`",
    "Updated the `RunPullRequestDescription` method to accept an additional parameter `diffInputFile`",
    "Implemented the `FetchDiff` method to handle fetching diff content from a file, URL, or git command based on the provided input"
  ]
}

If I copy to a file and run dotnet run pr description --outputFormat json -d "C:\Users\markkarle\Documents\testdiff.txt":

 {
  "Title": "Add diff input file option to PRCommand",
  "Summary": "This pull request adds a new option to the PRCommand, allowing users to specify a diff input file or URL. This makes it easier for users to generate pull request descriptions based on custom diff files or remote sources.",
  "Changes": [
    "Added a new option `--diffInputFile` (alias `-d`) to PRCommand, allowing users to specify a diff input file or URL",
    "Updated the `GeneratePRDescriptionCommand` method to include the new `diffInputFileOption`",
    "Modified the `RunPullRequestDescription` method to accept an additional `diffInputFile` parameter",
    "Implemented the `FetchDiff` method to handle fetching the diff content from a file, URL, or git command based on the provided `diffInputFile` value"
  ]
}

And finally if I leave out the option and run dotnet run pr description --outputFormat json

{
  "Title": "Add diff input file option to PRCommand",
  "Summary": "This pull request adds a new option to the PRCommand class, allowing users to specify a diff input file or URL. This new feature enhances the flexibility of the command and makes it more versatile for different use cases.",
  "Changes": [
    "Added a new option `--diffInputFile` (alias `-d`) to the PRCommand class, allowing users to specify a diff input file or URL.",
    "Updated the `GeneratePRDescriptionCommand` method to include the new `diffInputFileOption`.",
    "Modified the `RunPullRequestDescription` method to accept an additional parameter `diffInputFile` and use it when fetching the diff.",
    "Implemented the `FetchDiff` method to handle fetching the diff from a file, URL, or by running the `git show` command."
  ]
}