Add Stream loaders for BearSSL (#7675)

Fixes #7671

Allows for code to do things like read certs from LittleFS or even HTTP
connections with code like:

  File cert = LittleFS.open("/client-crt.pem", "r");
  clientCert = new X509List(cert, cert.size());
  cert.close();

libraries/ESP8266WiFi/src/BearSSLHelpers.cpp

@@ -626,6 +626,17 @@ namespace brssl {
     return pk;
   }
 
+  static uint8_t *loadStream(Stream& stream, size_t size) {
+    uint8_t *dest = (uint8_t *)malloc(size);
+    if (!dest) {
+      return nullptr;  // OOM error
+    }
+    if (size != stream.readBytes(dest, size)) {
+      free(dest);  // Error during read
+      return nullptr;
+    }
+    return dest;
+  }
 };
 
 
@@ -648,6 +659,15 @@ PublicKey::PublicKey(const uint8_t *derKey, size_t derLen) {
   parse(derKey, derLen);
 }
 
+PublicKey::PublicKey(Stream &stream, size_t size) {
+  _key = nullptr;
+  auto buff = brssl::loadStream(stream, size);
+  if (buff) {
+    parse(buff, size);
+    free(buff);
+  }
+}
+
 PublicKey::~PublicKey() {
   if (_key) {
     brssl::free_public_key(_key);
@@ -711,6 +731,15 @@ PrivateKey::PrivateKey(const uint8_t *derKey, size_t derLen) {
   parse(derKey, derLen);
 }
 
+PrivateKey::PrivateKey(Stream &stream, size_t size) {
+  _key = nullptr;
+  auto buff = brssl::loadStream(stream, size);
+  if (buff) {
+    parse(buff, size);
+    free(buff);
+  }
+}
+
 PrivateKey::~PrivateKey() {
   if (_key) {
     brssl::free_private_key(_key);
@@ -781,6 +810,17 @@ X509List::X509List(const uint8_t *derCert, size_t derLen) {
   append(derCert, derLen);
 }
 
+X509List::X509List(Stream &stream, size_t size) {
+  _count = 0;
+  _cert = nullptr;
+  _ta = nullptr;
+  auto buff = brssl::loadStream(stream, size);
+  if (buff) {
+    append(buff, size);
+    free(buff);
+  }
+}
+
 X509List::~X509List() {
   brssl::free_certificates(_cert, _count); // also frees cert
   for (size_t i = 0; i < _count; i++) {

libraries/ESP8266WiFi/src/BearSSLHelpers.h

@@ -43,6 +43,8 @@ class PublicKey {
     PublicKey();
     PublicKey(const char *pemKey);
     PublicKey(const uint8_t *derKey, size_t derLen);
+    PublicKey(Stream& stream, size_t size);
+    PublicKey(Stream& stream) : PublicKey(stream, stream.available()) { };
     ~PublicKey();
 
     bool parse(const char *pemKey);
@@ -69,6 +71,8 @@ class PrivateKey {
     PrivateKey();
     PrivateKey(const char *pemKey);
     PrivateKey(const uint8_t *derKey, size_t derLen);
+    PrivateKey(Stream& stream, size_t size);
+    PrivateKey(Stream& stream) : PrivateKey(stream, stream.available()) { };
     ~PrivateKey();
 
     bool parse(const char *pemKey);
@@ -98,6 +102,8 @@ class X509List {
     X509List();
     X509List(const char *pemCert);
     X509List(const uint8_t *derCert, size_t derLen);
+    X509List(Stream& stream, size_t size);
+    X509List(Stream& stream) : X509List(stream, stream.available()) { };
     ~X509List();
 
     bool append(const char *pemCert);