Improve service registration

[Tyrrrz]

JIRA item: https://bitwarden.atlassian.net/browse/PAS-206

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Based on the items raised in the "general feedback" meeting with me and @abergs.

Suggestion 1: rename AddPasswordlessSdk() to AddPassworless(). The Identity integration should either provide its own overloads with the same name (ideal) or have a separate method, like AddPasswordlessIdentity() (less ideal).

Suggestion 2: AddPasswordlessSdk() should either be callable without arguments (in which case the ApiKey/ApiSecret are resolved from conventional environment variables), or the options => ... overload should throw an exception when those two values aren't provided.