Fix warnings

[Tyrrrz]

• Fixed the warnings related to docs and some other minor issues.
• Added missing using statements in PasswordlessClient.
• Tried to optimize constructor calls in PasswordlessClient to avoid repetition.

I also want to note that the current wiring of HttpClient in PasswordlessClient is not very good. It mutates the state of the client (i.e. sets the BaseUrl property and adds default headers). This is suboptimal because clients can be reused and changing those properties may cause issues in other parts of the program.

There are two recommended approaches.

1. (Basic) Take the HttpClient as is and instead set all the required items on HttpRequestMessage every time it's created. This can be encapsulated in a helper method.
2. (Complex) Create a PasswordlessHttpHandler that will wire the correct headers and resolve proper URLs. It can then take the upstream HttpClient or another HttpMessageHandler in its delegation chain. This is too complex for this situation but might be reasonable if we want to add things like custom retry handling (for example on 429 responses).

This is an example of what the complex approach may look like:

• https://github.com/Tyrrrz/YoutubeExplode/blob/master/YoutubeExplode/YoutubeHttpHandler.cs
• https://github.com/Tyrrrz/YoutubeExplode/blob/master/YoutubeExplode/Utils/ClientDelegatingHandler.cs
• https://github.com/Tyrrrz/YoutubeExplode/blob/master/YoutubeExplode/YoutubeClient.cs#L43

[Tyrrrz]

@abergs @justindbaur I reverted the constructor changes because they warrant their own, larger PR. Please review the remaining changes so I can merge.

[abergs]

LGTM.