Allow environment for credentials in TestEntrypoint

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"
},
```