Add ability to only generate output invoke functions

[zbuchheit]

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Issue details

Add the ability to only generate output invoke functions/prevent the generation of plain invoke functions.

This may seem like an arbitrary request on its own, but the ultimate desire would be to mark outputs on an invoke as secret, and given plain invokes don't have a mechanism to mark responses as secrets, there is no way to enforce users to use the output invoke and ensure the response data is always marked as secret correctly.

Affected area/feature

Related to:#259

[iwahbe]

Right now, there is no way to express that a function should be output-only. This will require schema and codegen support from pulumi/pulumi.

Solving plain return values that are secret is a necessary pre-requisite to solving pulumi/pulumi#12710.