Add Bundled Exchanges parser interface

This adds an interface for parsing the Bundled Exchanges format into the
data_decoder service. The actual parsing code will be added in the next
CL (https://crrev.com/c/1626065).

Bug: 969596
Change-Id: I776199c8a55cd0137dc629ef6199ffa1959b9922
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1640524
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669585}