MINOR: htx: Add HTX_BLK_RAW_DATA block type

MINOR: htx: Add HTX_BLK_RAW_DATA block type

An HTX message is a partial and stateless representation of an HTTP
message. Tunneled data are not part of the HTTP message. However, for now,
they are stored in DATA blocks, exactly like the HTTP payload. So there is
no way to make the difference between both.

A dedicated block type, HTX_BLK_RAW_DATA, is added to fix this. For now, no
producer emits such block. Only the HTX API is able to handle it. From this
point of view, a RAW_DATA block is exactly the same as a DATA block. Both
carry a payload that may be partially consumed. htx_is_data_type() function
was added to test this property on a block type.

It is important to add the new type in htx_get_blk_value() and
htx_set_blk_value_len() switches. Both have a default case silently doing
nothing. So a missing entry would lead to an empty payload on one side and
to a size never updated on the other side.