epee: add SSL support

RPC connections now have optional tranparent SSL.

An optional private key and certificate file can be passed,
using the --{rpc,daemon}-ssl-private-key and
--{rpc,daemon}-ssl-certificate options. Those have as
argument a path to a PEM format private private key and
certificate, respectively.
If not given, a temporary self signed certificate will be used.

SSL can be enabled or disabled using --{rpc}-ssl, which
accepts autodetect (default), disabled or enabled.

Access can be restricted to particular certificates using the
--rpc-ssl-allowed-certificates, which takes a list of
paths to PEM encoded certificates. This can allow a wallet to
connect to only the daemon they think they're connected to,
by forcing SSL and listing the paths to the known good
certificates.

To generate long term certificates:

openssl genrsa -out /tmp/KEY 4096
openssl req -new -key /tmp/KEY -out /tmp/REQ
openssl x509 -req -days 999999 -sha256 -in /tmp/REQ -signkey /tmp/KEY -out /tmp/CERT

/tmp/KEY is the private key, and /tmp/CERT is the certificate,
both in PEM format. /tmp/REQ can be removed. Adjust the last
command to set expiration date, etc, as needed. It doesn't
make a whole lot of sense for monero anyway, since most servers
will run with one time temporary self signed certificates anyway.

SSL support is transparent, so all communication is done on the
existing ports, with SSL autodetection. This means you can start
using an SSL daemon now, but you should not enforce SSL yet or
nothing will talk to you.

tests/unit_tests/epee_utils.cpp

@@ -389,6 +389,25 @@ TEST(ToHex, String)
 
 }
 
+TEST(FromHex, String)
+{
+    // the source data to encode and decode
+    std::vector<uint8_t> source{{ 0x00, 0xFF, 0x0F, 0xF0 }};
+
+    // encode and decode the data
+    auto hex = epee::to_hex::string({ source.data(), source.size() });
+    auto decoded = epee::from_hex::vector(hex);
+
+    // encoded should be twice the size and should decode to the exact same data
+    EXPECT_EQ(source.size() * 2, hex.size());
+    EXPECT_EQ(source, decoded);
+
+    // we will now create a padded hex string, we want to explicitly allow
+    // decoding it this way also, ignoring spaces and colons between the numbers
+    hex.assign("00:ff 0f:f0");
+    EXPECT_EQ(source, epee::from_hex::vector(hex));
+}
+
 TEST(ToHex, Array)
 {
   EXPECT_EQ(