Allow to specify multiple runtimes

[Thorleon]

As AWS supports ARM architecture for 20% billing saving serverless-go-plugin should allow to compile single bootstrap file for provided.al2 runtime.

[hexadecy]

Works well with this config:

custom:
  go:
    supportedRuntimes: provided.al2
    buildProvidedRuntimeAsBootstrap: true
    cmd: GOARCH=arm64 GOOS=linux go build -tags lambda.norpc -ldflags="-s -w"

functions:
  helloWorld:
    runtime: provided.al2
    architecture: arm64
    handler: main.go

[Thorleon]

Thanks @hexadecy for checking!

Just FYI: I have changed config field name from buildAsBootstrap to buildProvidedRuntimeAsBootstrap, rest should work in the same manner.