Enable custom filter in AprsIsConnection and AprsSharp CLI

[CBielstein]

Description

In order to get customized server-side filtering, we need to expose the ability to customize the filter argument when logging in to an APRS-IS server. This PR addresses that by making filter an optional parameter on AprsIsConnection and allowing input in the AprsSharp CLI.

Note, this is a first part of #63, but does not complete all of it.

Changes

• Expose the filter argument in AprsIsConnection.Receive(string?, string?, string?)
• Add Console.ReadLine() input to take the filter on the command line

Validation

• Build and tests pass
• Manually validated by filtering to different locations and seeing different packets. See attached screenshot. Coordinates are Seattle's Space Needle, observe gridsquare CN87 in position reports

[CBielstein]

I assume that printing the prompt and input on the same line using Console.Write() is maybe to make it readable and avoid maybe unnecessarily going to a new line or is there any other reason for avoiding printing the prompt and input on a separate line using Console.WriteLine

@eddywine Yeah, I was just doing it for readability in the console app. I thought it looked nicer. Personal preference, I suppose. This should all change with #78 merging anyway.