m2m allow STS role session name to be overridden

[monde]

m2m allow STS role session name to be overridden with --aws-sts-role-session-name [value] CLI flag.

default - name is okta-aws-cli

$ okta-aws-cli m2m --exec -- aws sts get-caller-identity
{
    "UserId": "ABC:okta-aws-cli",
    "Account": "123",
    "Arn": "arn:aws:sts::123:assumed-role/myrole/okta-aws-cli"
}

--aws-sts-role-session-name - name is a-user

$ okta-aws-cli m2m --aws-sts-role-session-name a-user --exec -- aws sts get-caller-identity
{
    "UserId": "DEF:a-user",
    "Account": "123",
    "Arn": "arn:aws:sts::123:assumed-role/myrole/a-user"
}

Closes #165