feat: setup configuration with default fallbacks

Summary:

Now the users configuration and the default configuration is separated. This
will make it easier to setup ivy with some defaults. Right now we only have the
backends configurable however, it looks like we will be making other components
of ivy configurable.

To use this you can now call ivy.setup with no parameters and you will get
the default config. You can also call it with a partial config and if the
option is not found in the users config it will fallback to the users config
value.

Test Plan:

Manual testing and with unit tests in CI