Added loadCert methods to WiFiClientSecure

[lbernstone]

Fixes #1957.
Allows a file object to be used as a certificate in WiFiClientSecure.

[stickbreaker]

@lbernstone I have some question on these load* function using _loadStream().

from the original issue #1957 the OP said he used these three function to configure the secureClient:

In the ESP8266 I have stored a root CA certificate a device certificate and a private key in SPIFFS which I then load into WiFiClientSecure using the loadCACert, loadCertificate and loadPrivateKey methods which accept a Stream as a parameter and load from SPIFFS no problem.

My question is that _loadStream() uses a static char * for the stream buffer. So, every call to _loadStream() reuses the same pointer, It free's the buffer if necessary before each use, BUT prior calls to load* stored the address that is now being free'd? Am I totally confused(very possible).

If I understand the OP's usage, he will use:

WifiClientSecure client;
client.loadCACert(file);// which reads from Stream and stores into heap buffer address also held in static char *
client.loadCertificate(file2);// which frees buffer containing CACertificate? then reallocates buffer for Certificate
client.loadPrivateKey(file3); // which frees buffer containing Certificate?

Now:
client._CA_cert point to freed memory/invalid
client._cert points to freed memory/invalid
client.private_key points to private_key/valid

I am working #3679
Chuck.

[stickbreaker]

@lbernstone I'm thinking that these pointers need a type field, something to mark constant or heap. So that the destructor can clean up, right now the WifiSecureClient() destructor ignores these pointers.

Chuck.

[lbernstone]

Yes. Messy, messy. I'll be afk for a couple days, but will take a look this weekend. Maybe just needs the destructor to manually free the memory.