Allow environment for credentials in TestEntrypoint

[benbridts]

Currently I have to pass credentials to the TestEntryPoint by defining it in the event:

{
    "credentials": {
        "accessKeyId": "...",
        "secretAccessKey": "...",
        "sessionToken": "..."
    },
...

It would be nice if I could write something like:

{
    "credentials": {
        "credential_source": "Environment"
    },

that would make the _parse_test_requests function get the credentials from the environment. Another option could be the support something like this, that would use a specific profile:

{
    "credentials": {
        "credential_source": "Profile",
        "profile_name": "my-dev-profile"
    },