Shared environment variables for Lambda functions

### Is this related to a new or existing Amplify category?

function

### Is this related to another service?

Lambda function

### Describe the feature you'd like to request

Cloudformation has https://docs.aws.amazon.com/serverless-application-model/latest/developerguide/sam-specification-template-anatomy-globals.html where you can define environment variables for ALL your functions rather than having to attach them to the individual functions one-by-one

### Describe the solution you'd like

We should be able to configure a `globals` as well.

### Describe alternatives you've considered

None.

### Additional context

_No response_

### Is this something that you'd be interested in working on?

- [ ] 👋 I may be able to implement this feature request
- [ ] ⚠️ This feature might incur a breaking change