support Node 18 for functions

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

function

### Is this related to another service?

_No response_

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

https://aws.amazon.com/blogs/compute/node-js-18-x-runtime-now-available-in-aws-lambda/

Notable callout on sdk migration from v2 to v3
> **AWS SDK for JavaScript upgrade to v3**
> If your existing Lambda functions are using the included SDK v2, then you must update your function code to use the SDK v3 when upgrading to the Node.js 18 runtime. This is the recommended approach when upgrading existing functions to Node.js 18. Alternatively, you can use the Node.js 18 runtime without updating your existing code if you deploy the SDK v2 together with your function code.

### Describe the solution you'd like

see above

### Describe alternatives you've considered

n/a

### Additional context

_No response_

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

- [ ] 👋 I may be able to implement this feature request

### Would this feature include a breaking change?

- [ ] ⚠️ This feature might incur a breaking change

Start by tracing how the Amplify CLI selects the runtime for function categories and compare its existing Node.js runtime handling. Read the AWS Lambda Node.js 18 guidance linked in the issue, especially the AWS SDK v2-to-v3 compatibility note. Done means functions can use Node.js 18 while existing runtime support remains intact.

Written by the indexing model from the issue text.