Static Env ServerlessFramework Plugin

This plugin is is intended for use with Lambda@Edge to replace process.env and other
similar mechanisms referencing environment variables with static values so that environment
specific configuration can be used in Lambda@Edge (which does not support environment variables).

This is forked from serverless-plugin-embedded-env-in-code
with as it takes a different solution to the same problem of providing environment variables to Lambda@Edge.

In summary, what this does is take all environment variables specified in the serverless.yml in
provider.environment or function.environment and packages them into the deployment zip so they
become defaults if the environment variables defined when the function is executed. Practically speaking
this means any environment variables specified in provider.environment or function.environment are
available to Lambda@Edge functions the same way as they would to a normal lambda. The key difference being
a redeployment is required to update the values.

Configuration

Environment variables and their values are configured as normal, either in the top level
provider.environment
or function.engironment

When the function is packaged for deployment the values will be embedded in the package so that they are present in process.env
as normal. Note that any environment variables actually set in the environment will take precedence,
these are packaged as defaults only.

By default, any function with a cloudFront event trigger
will have it's environment variables embedded during packaging.

A function can explicitly opt in or out of this behavior by specifying the boolean includeStaticEnv in the function
definition.

Example

The below serverless.yml shows the various usages

``
service:
name: static-env-example

provider:
name: aws
runtime: nodejs12.x
stage: ${opt:stage, 'development'}
region: 'us-east-1'

# These environment variables will be made available to all function and included
# where static environment is embedded
environment:
SHARED_ENVIRONMENT_VAR1: 'value1'
SHARED_ENVIRONMENT_VAR2: 'value2'

plugins:
- serverless-plugin-static-env

functions:
edgeFunctionWithStaticEnv:
handler: dist/endpoints/edgeFunctionWithStaticEnv.handler
environment:
FUNCTION_SPECIFIC_ENVIRONMENT_VAR1: 'value3'
FUNCTION_SPECIFIC_ENVIRONMENT_VAR2: 'value4'
# includeStaticEnv: true # this will have static env by default because it includes a
# cloudFront event trigger. If this was set to false it would
# not have a static env even though it has a cloudFront trigger
#
# Without a cloudFront trigger this would have to be explicitly
# set to true to opt in to embedding the static environment
events:
- cloudFront:
eventType: viewer-response
origin: s3://bucketname.s3.amazonaws.com/files
`

How it Works

For each function an additional -env.js file is generated with the static environment. E.g. with
the example configuration above the file dist/endpoints/edgeFunctionWithStaticEnv-env.js would be
generated with the following content

`
process.env["SHARED_ENVIRONMENT_VAR1"] = process.env["SHARED_ENVIRONMENT_VAR1"] == null ? "value1": process.env["SHARED_ENVIRONMENT_VAR1"];
process.env["SHARED_ENVIRONMENT_VAR2"] = process.env["SHARED_ENVIRONMENT_VAR2"] == null ? "value2": process.env["SHARED_ENVIRONMENT_VAR2"];
process.env["FUNCTION_SPECIFIC_ENVIRONMENT_VAR1"] = process.env["FUNCTION_SPECIFIC_ENVIRONMENT_VAR1"] == null ? "value3": process.env["FUNCTION_SPECIFIC_ENVIRONMENT_VAR1"];
process.env["FUNCTION_SPECIFIC_ENVIRONMENT_VAR2"] = process.env["FUNCTION_SPECIFIC_ENVIRONMENT_VAR2"] == null ? "value4": process.env["FUNCTION_SPECIFIC_ENVIRONMENT_VAR2"];
`

The file dist/endpoints/edgeFunctionWithStaticEnv.js would also be updated to require dist/endpoints/edgeFunctionWithStaticEnv-env.js`
as it's first action ensuring that these variables are present before any other required code is executed.