Provide the ability to disable extension specific instructions in agent mode

[kvenkatrajan]

Allow for users to remove extension specific instructions for prompts. There are cases where extension specific instructions may not be applicable to a prompt. For example in this case below, Azure instructions are not applicable, it might be good to have the ability to remove them (similar to files in the context). here's an example reported by @timheuer -

Here's another ticket reported by a customer microsoft/GitHub-Copilot-for-Azure#470

[aeschli]

The Azure extension should not force instruction file onto the user
It must not use applyTo: '**'' but define no 'applyTo': The the instruction is loaded by the agent on demand