Add API for custom component theming with M3

[mmalerba]

A new API for this is necessary because the existing APIs (e.g. mat.get-color-from-palette) require the user to have some knowledge of how the theme object is structured. With the M3 theme objects, we made he decision that the theme object structure should be opaque to the user.

I will do a followup PR to add support for using the same API to access values from M2 theme objects. This will allow users to move their custom components on to the new API before updating to use an M3 theme.

With the new theming APIs, I'm focusing on trying to catch common errors a user might make and report helpful error messages rather than the $map: null is not a map errors that our previous APIs often threw. I've added tests to cover these error cases as well as the expected cases.

I'm introducing the APIs as private in this PR, but will do a followup once they have M2 theme support to make them public.

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