iceberg: add the example program

Port examples/iceberg.c from the source tree: a full Iceberg session
demonstrating the call order from the module docs -- distributed key
generation, pubshare_gen/pubkey_agg to obtain the group public key,
nonce_gen/nonce_agg into an ordinary MuSig2 public nonce, and
partial_sign/partial_sig_agg into an ordinary MuSig2 partial
signature.

One content adaptation: the secp256k1_musig_nonce_process call gains
a NULL adaptor argument, matching this repo's zkp musig variant.

Wired like the chilldkg example: autotools noinst_PROGRAMS +
TESTS entry under ENABLE_MODULE_ICEBERG (the example runs as part of
make check), CMake example target in examples/CMakeLists.txt, and
iceberg_example added to .gitignore.

Verified: ./iceberg_example runs to completion (exit 0) under both
build systems.

.gitignore

@@ -57,6 +57,7 @@ contrib/gh-pr-create.sh
 
 frost_example
 chilldkg_example
+iceberg_example
 
 ### CMake
 /CMakeUserPresets.json