Implement the ability to deprecate a CommandOption

[ChristosMaragkos]

This is in regards to issue #1314 in the main repository, which has not yet been migrated.

This PR introduces the ability to mark a CommandOption as Deprecated, marking it as obsolete and subject to removal. In this first iteration of my implementation, developers can deprecate CommandOptions simply by specifying a deprecation message in the CommandOptionAttribute ctor, like so:

[CommandOption("-d|--deprecated <VALUE>", isRequired: true, deprecationMessage: "This option is deprecated. Please use <...>")]
public string? Deprecated { get; set; }

Deprecated options are parsed and consumed properly, while also printing their deprecation message as a warning to the console.
A unit test is also included, which I (successfully) ran locally. It can be found in CommandOptionAttributeTests.Deprecation.cs.

[patriksvensson]

I think we should go the route of using the built-in ObsoleteAttribute here.

[ChristosMaragkos]

I'm working on it. The semantics this provides are much cleaner compared to an optional ctor parameter.
I'll probably have a revision ready before long.

[ChristosMaragkos]

This second iteration simply checks for the existence of ObsoleteAttribute on the backing property of the given CommandOption. The syntax is this:

[CommandOption("-d|--deprecated <VALUE>"]
[Obsolete("This option is deprecated. Please use <...>")]
public string? Deprecated { get; set; }