Support Session Tags when assuming a Role

**Is your feature request related to a problem? Please describe.**
Currently you can't use a named profile to assume a role, if you want to have session tags present on that role.

**Describe the solution you'd like**
It would be great if I could use an environment variable (and config option) to also add session tags to an assume role:

```bash
AWS_SESSION_TAGS='ChangeTicket=CT-1234' AWS_DEFAULT_PROFILE='profile' aws sts get-caller-identity
# or
AWS_SESSION_TAGS='ChangeTicket=CT-1234' aws sts get-caller-identity --profile profile
# or
aws sts get-caller-identity --profile profile --cli-session-tags ChangeTicket=CT-1234 --profile profile
```

**Describe alternatives you've considered**
Currently you can either do the STS call yourself as en external process (which is annoying), or use a third party tool (which might not be wanted)