feat(sso_credentials): if new sso session format in config, use sso token provider in sso credentials

[kuhe]

• in SSO Credentials, use the SSO Token Provider to get the token string if the config format is in the new format with an sso-session name.

Checklist

• npm run test passes
• n/a .d.ts file is updated
• changelog is added, npm run add-change
• run bundle exec rake docs:api and inspect doc/latest/index.html if documentation is changed
• n/a run npm run integration if integration test is changed
• n/a non-code related change (markdown/git settings etc)

testing:

sso.js

var AWS = require('../../lib/aws');
var S3 = require('../../clients/s3');

Promise.resolve()
  .then(function () {
    return new Promise(function (resolve) {
      var creds = new AWS.SsoCredentials({
        callback: function (err, done) {
          resolve(err || creds);
        }
      });
    });
  })
  .then(function (credentials) {
    console.log('credentials from:', credentials.profile);
    return new S3({credentials: credentials, region: 'us-east-1'}).listBuckets({}).promise();
  })
  .then(function (data) {
    console.log('list buckets OK');
  })
  .catch(function (err) {
    console.error('oh no', err);
  });

test:
	make legacy
	make new

legacy:
	AWS_SDK_LOAD_CONFIG=1 AWS_PROFILE=sso-legacy node sso

new:
	AWS_SDK_LOAD_CONFIG=1 AWS_PROFILE=sso-token node sso

~/.aws/config

[profile sso-legacy]
sso_account_id = ****
sso_region = us-east-1
sso_role_name = ******
sso_start_url = https://d-******.awsapps.com/start

[profile sso-token]
sso_session = dev
sso_account_id = ****
sso_role_name = ******

[sso-session dev]
sso_region = us-east-1
sso_start_url = https://d-******.awsapps.com/start
sso_registration_scopes = sso:account:access

[kuhe]

corresponding v3 PR: aws/aws-sdk-js-v3#4145