Add support for EVP_PKEY_CTX callback functions (aws#1905)

We tried to no-op these functions, but it turns out Ruby depends on them
pretty extensively as the interruption mechanism for threads. One of
Ruby's tests depends on `EVP_PKEY_CTX_get_app_data` to return an
actual value from the callback function, but we return NULL as a no-op. Ruby
seems to depend on the `EVP_PKEY` callback function and relevant
application data to correctly handle interruptions. Based on the relevant
commit messages, the expectation is that the operation is interrupted, but
AWS-LC continues resuming the operation and returns a generated RSA key.
It looks like we may have to consider implementing functionality for
these callback functions. This issue also applies to a test failure in
`test/openssl/test_pkey_dh.rb` and `test/openssl/test_pkey_dsa.rb`. We
probably aren't going to support DSA, but this will need to be applied
to DH somewhere down the line.

* Commits:
* ruby/openssl@88b90fb
* ruby/ruby@d3507e3

new test that verifies this works with `EVP_PKEY_RSA`

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license and the ISC license.

crypto/fipsmodule/evp/internal.h

@@ -255,7 +255,12 @@ int EVP_RSA_PKEY_CTX_ctrl(EVP_PKEY_CTX *ctx, int optype, int cmd, int p1, void *
 #define EVP_PKEY_CTX_KEYGEN_INFO_COUNT 2
 
 // EVP_PKEY_CTX_KEYGEN_INFO_COUNT is the maximum array length for
-// |EVP_PKEY_CTX->keygen_info|.
+// |EVP_PKEY_CTX->keygen_info|. The array length corresponds to the number of
+// arguments |BN_GENCB|'s callback function handles.
+//
+// |ctx->keygen_info| map to the following values in |BN_GENCB|:
+//     1. |ctx->keygen_info[0]| -> |event|
+//     2. |ctx->keygen_info[1]| -> |n|
 #define EVP_PKEY_CTX_KEYGEN_INFO_COUNT 2
 
 struct evp_pkey_ctx_st {