Add pelican_url package to standardize Pelican URL parsing/metadata d…

…iscovery

The package is meant to feel very similar to `net/url`, but provides utilities
specific to pelican. For example, upon parsing a pelican url, it can help with:
- validating query parameters
- federatin metadata discovery
- URL caching
- overall standardization of the way we handle pelican/osdf/stash URLs