feat: support source deploy option

[matthewaptaylor]

Description

This PR adds an option to the Firebase CLI to override the functions.source config option on firebase deploy. This is much like the option which overrides the public directory when deploying to Hosting.

The use case is where a user deploys a functions package whose structure is not supported by Firebase, such as a monorepo package. When deploying a functions package in a monorepo, a common strategy is to create a directory which contains the functions and other internal monorepo dependencies, and deploy that. One such example is isolate-package. Other strategies are mentioned in #653.

Scenarios Tested

• Tested running firebase deploy --only functions --source apps/functions/deploy, which deploys the deploy directory.
• Tested running firebase deploy --only functions, which deploys the folder specified in firebase.json.

Sample Commands

firebase deploy --only functions --source apps/functions/deploy
firebase deploy --only functions -s apps/functions/deploy
firebase deploy --source apps/functions/deploy
firebase deploy -s apps/functions/deploy

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