Detect code generation request based on secret word

Problem

As described in Differentiate on incoming code suggestion reque... (&11161 - closed) we want to start experimenting with differentiating between code-generation and code-completion, as right now every code-suggestion request is treated as code-completion.

Desired Outcome

Any incoming prompt that starts with a comment followed by the secret phrase Gitlab Duo Generate: is treated as code generation and sent without the secret prhase to code-bison via the gateway.

Example:

// Gitlab Duo Generate: A function that adds two numbers

Info

We can use the endpoint pointed out in #421318 (comment 1507435335) for a first test.