libosmo-gtlv: add generic TLV de- and encoder

An all new TLV parser supporting:

- Any size of T and L (determined by callback function),
- "Grouped IEs", so that an IE payload is a nested IE structure,
- optional/mandatory/multi-occurence IEs,
- decoding unordered tags (or enforcing strict order).

Will be used for PFCP message decoding and encoding, a T16L16V protocol
which requires above features.

Upcoming patches add
- translating PDUs to plain C structs and vice versa
- TLV generator to reduce repetition a in protocol definition
- TLIV capability

Previously, the way we deal with TLVs causes a lot of code
re-implementation: the TL decoding is taken care of by the API, but for
encoding, we essentially re-implement each protocol and each encoded
message in the individual programs. This API is an improvement in that
we only once implement the TL coding (or just use osmo_t8l8v_cfg /
osmo_t16l16v_cfg), get symmetric de- and encoding of the TL, and only
need to deal with the value part of each IE.

The common pattern of
- store TL preliminarily,
- write V data and
- update L after V is complete
is conveniently done by osmo_gtlv_put_update_tl().

Related: SYS#5599
Change-Id: Ib0fd00d9f288ffe13b7e67701f3e47073587404a
---
M configure.ac
M include/osmocom/Makefile.am
A include/osmocom/gtlv/Makefile.am
A include/osmocom/gtlv/gtlv.h
M src/Makefile.am
A src/libosmo-gtlv/Makefile.am
A src/libosmo-gtlv/gtlv.c
M tests/Makefile.am
A tests/libosmo-gtlv/Makefile.am
A tests/libosmo-gtlv/gtlv_test.c
A tests/libosmo-gtlv/gtlv_test.ok
M tests/testsuite.at
12 files changed, 1,258 insertions(+), 0 deletions(-)