Add Jsonnet native functions

[fujiwara]

This PR introduces Jsonnet native functions!

Add functions to go-jsonnet's native function, similar to those previously provided by text/template functions.

Examples

# ecspresso.yml
region: '{{ must_env `AWS_REGION` }}'
cluster: '{{ env `CLUSTER` `default` }}'
# ...

// ecspresso.jsonnet
local env = std.native('env');
local must_env = std.native('must_env');
{
  region: must_env('AWS_REGION'),
  cluster: env('CLUSTER', 'default'),
  // ...
}

// ecs-service-def.jsonnet
local env = std.native('env');
local tfstate = std.native('tfstate');
local vpc_tfstate = std.native('vpc_tfstate'); // func_prefix: vpc_
{
  desiredCount: std.parseInt(env('DESIRED_COUNT', '1')),
  loadBalancers: [
    {
      containerName: 'nginx',
      containerPort: 80,
      targetGroupArn: tfstate('aws_lb_target_group.nginx.arn'),
    },
  ],
  networkConfiguration: {
    awsvpcConfiguration: {
      securityGroups: [ vpc_tfstate('aws_security_group.foo.id') ],
      subnets: [
        vpc_tfstate('aws_subnet.az-a.id'),
        vpc_tfstate('aws_subnet.az-b.id'),
      ],
    },
  },

  // ...

// ecs-task-def.jsonnet
local ssm = std.native('ssm');
local secretsmanager_arn = std.native('secretsmanager_arn');
{
  containerDefinitions: [
    {
      environment: [
        {
          name: 'FOO',
          value: ssm('/ecspresso-test/foo'),
        },
      ],
      secrets: [
        {
          name: 'BAZ',
          valueFrom: secretsmanager_arn('ecspresso-test/baz'),
        },
      ],
//....

Add functions

• env(name, default)
• must_env(name)
• ssm(name)
• ssm_list(name, index)
• secretsmanager_arn(id)
• cfn_export(name)
• cfn_output(stackName, outputKey)
• tfstate(addr)

Limitations

env and must_env are available at first (for reading a config file).
Other functions will be available to load task definitions or service definitions.

This limitation is same to text/template functions.