Add credential_process support for assume-role

**Is your feature request related to a problem? Please describe.**
I'm trying to configure profile with role_arn and credential_process:
```
[profile test]
region = eu-west-1
role_arn = arn:aws:iam::123456789012:role/TestRole
credential_process = some-credential-process
```
but I got an error:
```
Partial credentials found in assume-role, missing: source_profile or credential_source
```
so I have to use a workaround
```
[profile test]
region = eu-west-1
role_arn = arn:aws:iam::123456789012:role/TestRole
source_profile = test-credential

[profile test-credential]
credential_process = some-credential-process
```

**Describe the solution you'd like**
Support credential_process with role_arn in one profile

**Additional context**
I'm using aws-cli/2.1.32