trace: support for serializing variable-length sequences of bytes

Tracepoint argument which extends 'blob_tag' will be interpreted as a
range of byte-sized values. Storage required to serialize such object
is proportional to its size.

I need it to implement storage-fiendly packet capturing using tracing layer.

It could be also used to capture variable length strings. Current
limit (50 chars) is too short for some paths passed to vfs calls. With
variable-length encoding, we could set a more generous limit.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>