barectf: add support for packet sequence number

barectf: add support for packet sequence number

Adds a new optional packet features object `sequence-number-field-type`,
which generates the metadata field type name `packet_seq_num` when
defined (defaults to false).

Currently this is conditionally maintained in `struct barectf_ctx` as a
`uint32_t` type like `events_discarded`.

In the future this could be improved to use the config type defined by
the stream in `struct barectf_default_ctx` so that the types are
aligned.

[Philippe Proulx: add documentation to Jon's patch.]

Refs: https://github.com/efficios/barectf/pull/24
Closes: https://github.com/efficios/barectf/issues/23
Signed-off-by: Jon Lamb <jon@auxon.io>
Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Ibfa9e4ab73a7c338e41377aae01d498dee62086b