Better initialization of relic

[schanzen]

We should either add a bbs_init function that initializes the subsystem or a compiler attribute (constructor) that does that when the library is loaded.
Currently, it is not even really documented outside of the tests how to use libbbs correctly.

[schanzen]

Constructor attribute use: https://git.gnunet.org/gnunet.git/tree/src/lib/util/crypto_random.c#n363
Example where init is used: https://github.com/Fraunhofer-AISEC/libbbs/blob/main/test/bbs_e2e_sign_n_proof.c#L6
Maybe constructor with bbs_init as portability fallback?