Stop including the embedded OctopusCLI

[johnsimons]

Up till v4 of this extension we have been shipping as part of the vsix extension the embedded version of OctopusCLI.

This has the benefit of letting the user use our tasks without having to remember to install the OctopusCLI beforehand.

However this is also an antipattern,

• It locks customers on a specific version of OctopusCLI
• It makes the vsix package larger and therefore very difficult to keep it under the 25MB limit of TFS 2018
• It is not what MS recommends, see https://github.com/microsoft/azure-pipelines-tool-lib/blob/master/docs/overview.md
• It requires .NET Core 2.0/3.1 to be installed on the agent

So in v5 the Octopus CLI Installer task downloads a self-contained (not dependant on .Net Core being installed on the agent) version of the Octopus CLI. We also continue to support having the Octopus CLI pre-installed in the agent and added to the PATH environment variable.

[johnsimons]

Fixed in https://github.com/OctopusDeploy/OctoTFS/pull/223