bundle-uri: create bundle_list struct and helpers

It will likely be rare where a user uses a single bundle URI and expects
that URI to point to a bundle. Instead, that URI will likely be a list
of bundles provided in some format. Alternatively, the Git server could
advertise a list of bundles.

In anticipation of these two ways of advertising multiple bundles,
create a data structure that represents such a list. This will be
populated using a common API, but for now focus on what data can be
represented.

Each list contains a number of remote_bundle_info structs. These contain
an 'id' that is used to uniquely identify them in the list, and also a
'uri' that contains the location of its data. Finally, there is a strbuf
containing the filename used when Git downloads the contents to disk.

The list itself stores these remote_bundle_info structs in a hashtable
using 'id' as the key. The order of the structs in the input is
considered unimportant, but future modifications to the format and these
data structures will place ordering possibilities on the set. The list
also has a few "global" properties, including the version (used when
parsing the list) and the mode. The mode is one of these two options:

1. BUNDLE_MODE_ALL: all listed URIs are intended to be combined
   together. The client should download all of the advertised data to
   have a complete copy of the data.

2. BUNDLE_MODE_ANY: any one listed item is sufficient to have a complete
   copy of the data. The client can choose arbitrarily from these
   options. In the future, the client may use pings to find the closest
   URI among geodistributed replicas, or use some other heuristic
   information added to the format.

This API is currently unused, but will soon be expanded with parsing
logic and then be consumed by the bundle URI download logic.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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