Implement the bundles-containing API endpoint.

Thanks for adding these comments. I'd be tempted to stick to the usual style,
with white space between the entries. Here are some suggestions
for slightly different phrasing:

    // name names the meta endpoint.
    name string

    // exclusive specifies whether the endpoint is
    // valid for charms only (charmOnly), bundles only (bundleOnly)
    // or to both (zero).
    exclusive int

    // get returns the expected data for the endpoint.
    get metaEndpointExpectedValueGetter