fix(aws-lambda): proper handling of custom runtimes with layers

[coderbyheart]

This compares layer and function runtime by name.

In case we construct a lambda and a layer with new Runtime(...) the check will fail because it compares two different object instances

 
    const baseLayer = new LayerVersion(this, 'layer', {
            compatibleRuntimes: [new Runtime('nodejs10.x')],
        });

    const l = new Function(this, 'lambda', {
            runtime: new Runtime('nodejs10.x'),
            layers: [baseLayer],
        });

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Since this is not changing any features / behavior I don't think we need to update tests.

✔️ By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license.

[spg]

Hey! I've already opened a pull request that kind of addresses the same concerns: #2543

[rix0rrr]

Just merged the other one, so I will be closing this. Thanks for the contribution though!