Add gnuTLS support to module

[danieldonoghue]

Allow use of either GnuTLS or OpenSSL for crypto functions in the module. (defaults to OpenSSL).

[kjdev]

I couldn't build it in my development environment or CI, so I used it as a reference and tried to deal with it.

https://github.com/kjdev/nginx-auth-jwt/tree/feat/gnutls-support

I have not been able to pass the test yet, so I need to make some adjustments.

t/auth_jwt.t ................. ok
t/auth_jwt_claim.t ........... ok
t/auth_jwt_claim_set.t ....... ok
t/auth_jwt_claims.t .......... ok
t/auth_jwt_key_file.t ........ ok
t/auth_jwt_key_request.t ..... ok
t/auth_jwt_leeway.t .......... ok
t/auth_jwt_phase.t ........... ok
t/auth_jwt_validate_alg.t .... 
Dubious, test returned 15 (wstat 3840, 0xf00)
Failed 15/176 subtests 
t/auth_jwt_validate_aud.t .... ok
t/auth_jwt_validate_exp.t .... ok
t/auth_jwt_validate_iat.t .... ok
t/auth_jwt_validate_iss.t .... ok
t/auth_jwt_validate_nbf.t .... ok
t/auth_jwt_validate_nonce.t .. ok
t/auth_jwt_validate_sig.t .... ok
t/auth_jwt_validate_sub.t .... ok
t/bearer_token.t ............. ok

Test Summary Report
-------------------
t/auth_jwt_validate_alg.t  (Wstat: 3840 (exited 15) Tests: 176 Failed: 15)
  Failed tests:  97-101, 118-122, 139-143
  Non-zero exit status: 15
Files=18, Tests=977, 1144 wallclock secs ( 0.15 usr  0.05 sys +  3.71 cusr  2.19 csys =  6.10 CPU)

[danieldonoghue]

I did manage to get this working and used it for some internal testing but I ended up abandoning it in favour of using the openssl version on a more recent os distro.

Im considering declining this PR but if you wanted to carry on working on it, feel free :)