ipn: generate LoginProfileView and use it instead of *LoginProfile where appropriate

Conventionally, we use views (e.g., ipn.PrefsView, tailcfg.NodeView, etc.) when
dealing with structs that shouldn't be mutated. However, ipn.LoginProfile has been
an exception so far, with a mix of passing and returning LoginProfile by reference
(allowing accidental mutations) and by value (which is wasteful, given its
current size of 192 bytes).

In this PR, we generate an ipn.LoginProfileView and use it instead of passing/returning
LoginProfiles by mutable reference or copying them when passing/returning by value.
Now, LoginProfiles can only be mutated by (*profileManager).setProfilePrefs.

Updates #14823

Signed-off-by: Nick Khyl <nickk@tailscale.com>
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