feat(auth): ability to add additional request headers

[evtkhvch]

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Short description of what this resolves:

Closes #1232

add headers?: HttpHeaders | { [header: string]: string | string[]; }; field to NbAuthStrategyOptions

BREAKING CHANGE:
NbOAuth2AuthStrategy.buildAuthHeader return type changed from any to HttpHeaders | undefined. If the method is overridden in a derived class, update its return type as well.