Rework CommandService#ExecuteAsync logic and allow easier extensibility.

[alikindsys]

In the current state, ExecuteAsync does two functions. Command Execution and Command Validation (Valid command check/ predicate check/ parameter check/ override check, etc.).
My idea is to split those into two separate functions, one responsible for validating the commands and another for execution.
For compatibility and no major breaks, i decided to keep the current public api unchanged with only the addition of a ValidateAndGetBestMatch function with the following signature.

public async Task<(IResult, Optional<CommandMatch>)> ValidateAndGetBestMatch(SearchResult matches,
ICommandContext context,
IServiceProvider provider,
MultiMatchHandling multiMatchHandling = MultiMatchHandling.Exception)

This function is to be called with Search(String)'s output making ExecuteAsync's logic exclusive for execution,
Search's exclusive for searching and Validate... exclusive for validation and command matching.

An intended side-effect is the extensibility this would allow if accepted. An external CommandService wrapper could leverage Discord.Net's Validate... in order to get the command that best fits without executing it, as the only current way of getting the command that was called is via an event handler, but there is no clean way for getting a command that will be called.

class Wrapper {
    //Task CheckForWhatever(string)
    var search = _commandService.Search(message.Content.Substring(argPos));
    var (status, command) = await ValidateAndGetBestMatch(search); //Other params ommited for sake of clarity
    if (!status.IsSucess) return;
    command.Value.ExecuteAsync(command.Command, context, status); 
}

Status

• Implemented on siscodeorg/Discord.Net
• Discussed by community on the discord channel. got mixed reactions.
• PR opened - Implementation of ValidateAndGetBestMatch #1700