Add support for environment variables

[Angelmmiguel]

Environment variables will be configured in TOML files. Every worker will define its own set of environment variable, which will be accessible only for them.

As a tentative API, we will include a new .env resource in the TOML config file:

name = "test"
version = "1"

[env]
MY_ENV_VAR = "Testing!"

Then, the environment variables will be passed to the inner modules via WASI. The environment variables will be accessible via default env methods from the different languages.

[Angelmmiguel]

We will be able to inject environments variables by using setting the variable value to $XXX:

name = "test"
version = "1"

[env]
TOKEN = "$TOKEN"
MY_ENV_VAR = "VALUE"

In the future, we may add a more complete definition of variables. However, for now we will go with a single section to keep it simple.

name = "test"
version = "1"

[env]
MY_ENV_VAR = "VALUE"

[env.from]
environment = ["TOKEN"]
dotenv = ".env"