Allow to set custom handler

[nalejandroveron]

When using Builder from @sls-next/lambda-at-edge there's a really useful functionality to add an extra handler (in case you want to call callbackWaitsForEmptyEventLoop = false, set any tracer or establish a DB connection, really useful!).

But unfortunately it's kinda useless on CDK because the default handler is stuck at index.handler, this PR solves that, so you can do the following:

export function MyStack(scope: Construct, id: string, props?: cdk.StackProps) {
  /** other things */

  new NextJSLambdaEdge(stack, `${id}-next`, {
    memory: 1536,
    runtime: Runtime.NODEJS_14_X,
    handler: 'handler.handler' // <------ We could replace the default handler and make use of the one we provided to "build"
  })
}

const builder = new Builder('.', './build', {
  args: ['build'], 
  handler: 'handler.js', // <------ We told builder we want this handler, but we should tell the lambda too ☝🏻
})

builder
  .build()
  .then(() => {
    const app = new cdk.App()
    MyStack(app, `example-stack`)
  })
  .catch((e) => {
    console.log(e)
    process.exit(1)
  })

[slsnextbot]

Handler Size Report

No changes to handler sizes.

Base Handler Sizes (kB) (commit e079765)

{
    "Lambda": {
        "Default Lambda": {
            "Standard": 1578,
            "Minified": 692
        },
        "Image Lambda": {
            "Standard": 1542,
            "Minified": 830
        }
    },
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1588,
            "Minified": 698
        },
        "Default Lambda V2": {
            "Standard": 1580,
            "Minified": 694
        },
        "API Lambda": {
            "Standard": 634,
            "Minified": 318
        },
        "Image Lambda": {
            "Standard": 1550,
            "Minified": 835
        },
        "Regeneration Lambda": {
            "Standard": 1233,
            "Minified": 566
        },
        "Regeneration Lambda V2": {
            "Standard": 1307,
            "Minified": 596
        }
    }
}

New Handler Sizes (kB) (commit 0221d1d)

{
    "Lambda": {
        "Default Lambda": {
            "Standard": 1578,
            "Minified": 692
        },
        "Image Lambda": {
            "Standard": 1542,
            "Minified": 830
        }
    },
    "Lambda@Edge": {
        "Default Lambda": {
            "Standard": 1588,
            "Minified": 698
        },
        "Default Lambda V2": {
            "Standard": 1580,
            "Minified": 694
        },
        "API Lambda": {
            "Standard": 634,
            "Minified": 318
        },
        "Image Lambda": {
            "Standard": 1550,
            "Minified": 835
        },
        "Regeneration Lambda": {
            "Standard": 1233,
            "Minified": 566
        },
        "Regeneration Lambda V2": {
            "Standard": 1307,
            "Minified": 596
        }
    }
}

[codecov]

Codecov Report

Merging #2409 (0221d1d) into master (e079765) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2409   +/-   ##
=======================================
  Coverage   83.59%   83.59%           
=======================================
  Files         102      102           
  Lines        3706     3707    +1     
  Branches     1188     1189    +1     
=======================================
+ Hits         3098     3099    +1     
  Misses        596      596           
  Partials       12       12           

Impacted Files	Coverage Δ
...rless-components/nextjs-cdk-construct/src/index.ts	93.96% <100.00%> (+0.05%)	⬆️

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