cpp-common/bt2: add `CommonBlobField` and `CommonDynamicBlobField`

cpp-common/bt2: add `CommonBlobField` and `CommonDynamicBlobField`

Use `CommonBlobField::data()` to access the data pointer and data length
(bytes) of the BLOB field as a `bt2s::span`. A `BlobField::data()`
returns `bt2s::span<std::uint8_t>` while `ConstBlobField::data()`
returns `bt2s::span<const std::uint8_t>`.

Use `CommonDynamicBlobField::length()` to set the length of a dynamic
BLOB field (like `CommonDynamicArrayField::length()`).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I3d9f9417e262a5fbbc9b7bb8f3d690d6628f7988
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12713