Lambda throttling (reserved concurrency)

## Prerequisites

* [x] I am running the latest version. (`up upgrade`)
* [x] I searched to see if the issue already exists.
* [ ] I inspected the verbose debug output with the `-v, --verbose` flag.
* [x] Are you an Up Pro subscriber?

## Description
Add `lamda.reserved_concurrency` to `up.json`.

[AWS announced reserved concurrency in November 2017](https://aws.amazon.com/about-aws/whats-new/2017/11/set-concurrency-limits-on-individual-aws-lambda-functions/). This "reservation" allows you to limit the number of concurrent executions of a lambda function. This is helpful for small projects which are using underpowered RDS databases with limited connections.

I took a brief glance into the GO AWS SDK. Reserved currency is not defined as a value in [CreateFunctionInput](https://docs.aws.amazon.com/sdk-for-go/api/service/lambda/#CreateFunctionInput) and is instead a separate call to [PutFunctionConcurrency](https://docs.aws.amazon.com/sdk-for-go/api/service/lambda/#Lambda.PutFunctionConcurrency).

According to the documentation it just returns a `429` error when throttling requests from API Gateway.

## Steps to Reproduce
N/A