Improve service registration

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.

Start at the AddPasswordlessSdk() service-registration entry point and trace how the ApiKey and ApiSecret options are currently supplied. Clarify the intended naming and Identity integration approach, then define the expected behavior for missing credentials and confirm the chosen registration API is covered by the relevant tests.

Written by the indexing model from the issue text.