Implement Federated Bundle Fetching

[azdagron]

This issue is to implement fetching a bundle from the SPIFFE Bundle Endpoint as defined in the [SPIFFE Trust Domain and Bundle] (https://github.com/spiffe/spiffe/blob/master/standards/SPIFFE_Trust_Domain_and_Bundle.md#5-spiffe-bundle-endpoint) specification. The v2-api branch has scaffolding for the implementation in fetch.go.

The code should implement both WebPKI and SPIFFE Authentication (the latter using the tlsconfig package implementation from #64).

This issue depends on #55, #59, and #64.