wip: hacky external crypto provider PoC

The "hacky"ness is mostly that I've only updated the `pkg-config` based
build infra.

The main changes:

* Unconditionally using the provider in `client.c` when using the
  `AWS_LC_RS` backend define.
* Unconditionally adding `pkg-config --libs rustls-post-qunatum-ffi` to
  the pkg config Makefile.

It's assumed the user will have built + installed
`rustls-post-quantum-ffi` to the relevant pkg-config/LD_LIBRARY_PATH
locations.

This is enough to demo that the system works :)

Makefile.pkg-config

@@ -49,7 +49,7 @@ target/%.o: tests/%.c tests/common.h | target
 	$(CC) -o $@ -c $< $(CFLAGS) $(shell pkg-config --cflags rustls)
 
 target/client: target/client.o target/common.o
-	$(CC) -o $@ $^ $(LDFLAGS) $(shell pkg-config --libs rustls)
+	$(CC) -o $@ $^ $(LDFLAGS) $(shell pkg-config --libs rustls) $(shell pkg-config --libs rustls-post-quantum-ffi)
 
 target/server: target/server.o target/common.o
 	$(CC) -o $@ $^ $(LDFLAGS) $(shell pkg-config --libs rustls)

tests/client.c

@@ -19,6 +19,7 @@
 
 /* rustls.h is autogenerated in the Makefile using cbindgen. */
 #include "rustls.h"
+#include "rustls-post-quantum-ffi.h"
 #include "common.h"
 
 /*
@@ -443,7 +444,8 @@ main(int argc, const char **argv)
 #endif
 
 #if defined(DEFINE_AWS_LC_RS)
-  default_provider = rustls_aws_lc_rs_crypto_provider();
+  default_provider = (const struct rustls_crypto_provider *)
+    rustls_post_quantum_ffi_crypto_provider();
 #elif defined(DEFINE_RING)
   default_provider = rustls_ring_crypto_provider();
 #else