[Parse incomplete chunks 3/9] Add `ChunkInfo` metadata to DS buffer to record (un)filled gap locations

[benkilimnik]

Summary: Adds metadata to the always_contiguous_data_stream_buffer to keep track of gap and filler locations. This forms the basis of tracking the number of bytes rendered unparseable due to the presence of a gap (i.e. the bytes cut off before the gap). It is also used to lazily parse as far as possible up to the gap in a future PR.

A TODO is added to the lazy_contiguous implementation in case we switch to that at some point in the future.

Note that this PR relies on changes introduced in #1786 and #1787.

Type of change: /kind feature

Test Plan: Test cases added to data_stream_buffer_test.cc

Additional Context: #1755