feat: Add SecureMediatorCommandExecutor

Issue: Move mediator commands into vscode-extension-we... (#310 - closed)

Description

From commit:

• This removes globally registered mediator comands when AuthProivder is available.
• The SecureMediatorCommandExecutor simply creates these commands in memory so it is secure.
• This MediatorCommandExecutor singleton management is so we can iteratively move away from these commands altogether.

Screenshot

No visual changes, but everything should still work:

web_ide_no_mediator_commands_demo

In integration using the package from this MR pipeline:

web_ide_oauth enabled	web_ide_oauth disabled