add MultiStrategyConfig type export

[thejaigoyal]

Description

Exporting MultiStrategyConfig type from src index so the type is available and can be imported for segregating the code and be used like below

import { MultiSamlStrategy, VerifyWithRequest, MultiSamlConfig } from 'passport-saml';

const multiSamlOptions: MultiSamlConfig = {
  passReqToCallback: true, // makes req available in callback
  getSamlOptions: async (request, done) => {
    const configuration = await findProvider(request);
    return done(null, configuration);
  },
};

export const multiSamlStrategy = new MultiSamlStrategy(multiSamlOptions, login);

Checklist:

• Issue Addressed: [ ]
• Link to SAML spec: [ ]
• Tests included? [ ]
• Documentation updated? [ ]

[thejaigoyal]

This PR belongs to Issue #674