[Plugin-API] Redundant 'CancellationToken' argument is present in each rpc-request

[vinokurig]

Description

On each rpc-request a new argument 'CancellationToken' is added to the arguments array:
https://github.com/theia-ide/theia/blob/2a2a6010329850e79b508b2133a62749d2d39e05/packages/plugin-ext/src/api/rpc-protocol.ts#L179-L181

It causes a wrong behavior of VsCode Git Extention. On executing some commands, the extension handles received arguments and 'CancellationToken' object influences on the commands result:
https://github.com/Microsoft/vscode/blob/7d39e805fb66a6eac22f19af8bb90f48da863fd5/extensions/git/src/commands.ts#L649-L677

The command handles resourceStates: SourceControlResourceState[] It converts the argument to undefined and adds to uri array. While iterating the array workspace.openTextDocument(uri) with undefined is applied: https://github.com/Microsoft/vscode/blob/7d39e805fb66a6eac22f19af8bb90f48da863fd5/extensions/git/src/commands.ts#L692

Related Commit: 4286ebd
Is needed for #4103

@akosyakov WDYT?

OS and Theia version:

Diagnostics:

[kittaakos]

Possible the same as #4310

[akosyakov]

ok, then we need to pass a boolean flag to indicate whether the cancellation token should be appended