asio: Use internal ssl context and engine impl

Implement asio-ssl layer with three classes in asio::ssl::mbedtls:
* context -- replaces SSL_CTX, used mainly as a container to options,
certs, keys
* engine -- replaces SSL, implements the actual mbedtls operations
* bio -- implements openssl BIO specifically tailered to mbedtls and
its asio usage

Further updates:
* asio: Used shared_ptr<> for bio pairs
* asio: Add error checks to mbedtls-bio
* asio: Address potential ssl-context ownership issue
* asio: Address potential bio-engine ownership issue


* Original commit: espressif/esp-idf@d823106aa6

components/asio/port/mbedtls/include/mbedtls_bio.hpp

@@ -0,0 +1,113 @@
+//
+// SPDX-FileCopyrightText: 2021 Espressif Systems (Shanghai) CO LTD
+//
+// SPDX-License-Identifier: BSL-1.0
+//
+#pragma once
+
+#include "asio/ssl/context_base.hpp"
+#include "asio/ssl/context.hpp"
+#include "sdkconfig.h"
+
+namespace asio {
+namespace ssl {
+namespace mbedtls {
+
+class bio {
+    static constexpr int BIO_SIZE = CONFIG_ASIO_SSL_BIO_SIZE;
+    static constexpr int BIO_FLAGS_READ = 1;
+    static constexpr int BIO_FLAGS_WRITE = 2;
+
+public:
+    int write(const void *buf, int len)
+    {
+        if (buf == nullptr || len <= 0) {
+            // not an error, just empty operation (as in openssl/bio)
+            return 0;
+        }
+        int remaining = size_ - offset_;
+        if (remaining <= 0) {
+            flags_ |= BIO_FLAGS_WRITE;
+            return -1;
+        }
+        int len_to_write = len > remaining ? remaining : len;
+        std::memcpy(&data_[offset_], buf, len_to_write);
+        offset_ += len_to_write;
+        dlen_ = offset_;
+        if (len_to_write == len) {
+            flags_ &= ~BIO_FLAGS_WRITE;
+        }
+        return len_to_write;
+    }
+
+    int read(void *buf, int len)
+    {
+        if (buf == nullptr || len <= 0) {
+            // not an error, just empty operation (as in openssl/bio)
+            return 0;
+        }
+        int remaining = peer_->dlen_ - peer_->roffset_;
+        if (remaining <= 0) {
+            flags_ |= BIO_FLAGS_READ;
+            return -1;
+        }
+        int len_to_read = remaining > len ? len : remaining;
+        std::memcpy(buf, &peer_->data_[peer_->roffset_], len_to_read);
+        peer_->roffset_ += len_to_read;
+        if (len_to_read == len) {
+            flags_ &= ~BIO_FLAGS_READ;
+        }
+        if (peer_->offset_) {
+            // shift data back to the beginning of the buffer
+            std::memmove(&peer_->data_[0], &peer_->data_[peer_->roffset_], peer_->offset_ - peer_->roffset_);
+            peer_->offset_ -= peer_->roffset_;
+            peer_->roffset_ = 0;
+            peer_->dlen_ = peer_->offset_;
+        }
+        return len_to_read;
+    }
+
+    size_t wpending() const
+    {
+        return dlen_ - roffset_;
+    }
+
+    size_t ctrl_pending()
+    {
+        return peer_->dlen_ - peer_->roffset_;
+    }
+
+    bool should_write() const
+    {
+        return flags_ & BIO_FLAGS_WRITE;
+    }
+
+    bool should_read() const
+    {
+        return flags_ & BIO_FLAGS_READ;
+    }
+
+    static std::pair<std::shared_ptr<bio>, std::shared_ptr<bio>>  new_pair(const char* error_location)
+    {
+        auto b1 = std::shared_ptr<bio>(new (std::nothrow) bio);
+        auto b2 = std::shared_ptr<bio>(new (std::nothrow) bio);
+        if (b1 == nullptr || b2 == nullptr) {
+            throw_alloc_failure(error_location);
+        } else {
+            b1->peer_ = b2;
+            b2->peer_ = b1;
+        }
+        return std::make_pair(b1, b2);
+    }
+
+private:
+    std::array<uint8_t, BIO_SIZE> data_ {};
+    size_t size_ {BIO_SIZE};
+    std::shared_ptr<bio> peer_ {nullptr};
+    int dlen_ {0};
+    size_t offset_ {0};
+    size_t roffset_ {0};
+    size_t flags_ {0};
+};
+
+} } } // namespace asio::ssl::mbedtls