Adding support for setting a custom root path for bundle loading

[sayzenberg]

Currently, RNW is configured to always load bundles using the "ms-appx:///Bundle/" URI and root path. There are some scenarios where this hardcoded value does not work; for example, when the bundle resides in an optional package, the URI needs to include the PFN.

This PR adds an option for developers to specify their own root path for bundle loading.

• DevSettings and ReactInstanceSettings now contain new fields for specifying the use of a custom path and the custom path itself.
• UwpReactInstance now contains a new helper to return the root path to classes that don't have references to DevSettings
• OInstance and ReactImage both use the custom root path when available.

Microsoft Reviewers: Open in CodeFlow

[akrantz]

Based on my previous comment, let's normalize with other work done for Office to allow overriding parts of the source bundle url (host/port/path/extension). If only the path component should be able to be normalized now, it could still use the same naming.

[ahimberg]

/azp run

[azure-pipelines]

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