Doc: examples how to load raw ECC keys

This does come up as usage questions occasionally (randombit#3137, randombit#4224)

src/examples/ecc_raw_private_key.cpp

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+#include <botan/bigint.h>
+#include <botan/ecdh.h>
+#include <botan/hex.h>
+#include <botan/pkcs8.h>
+#include <botan/rng.h>
+#include <botan/x509_key.h>
+
+#include <iostream>
+#include <string>
+
+int main() {
+   const std::string curve_name = "secp256r1";
+   const auto private_scalar_bytes =
+      Botan::hex_decode("D2AC61C35CAEE918E47B0BD5E61DA9B3A5C2964AB317647DEF6DFC042A06C829");
+
+   Botan::Null_RNG null_rng;
+   const auto domain = Botan::EC_Group::from_name(curve_name);
+   const auto private_scalar = Botan::BigInt(private_scalar_bytes);
+
+   // This loads the private scalar into an ECDH_PrivateKey. Creating an
+   // ECDSA_PrivateKey would work the same way.
+   const auto private_key = Botan::ECDH_PrivateKey(null_rng, domain, private_scalar);
+   const auto public_key = private_key.public_key();
+
+   std::cout << "Private Key (PEM):\n\n" << Botan::PKCS8::PEM_encode(private_key) << '\n';
+   std::cout << "Public Key (PEM):\n\n" << Botan::X509::PEM_encode(*public_key) << '\n';
+}

src/examples/ecc_raw_public_key.cpp

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+#include <botan/ecdsa.h>
+#include <botan/hex.h>
+#include <botan/x509_key.h>
+
+#include <iostream>
+#include <span>
+#include <string>
+#include <vector>
+
+std::vector<uint8_t> to_uncompress_point_encoding(std::span<const uint8_t> x, std::span<const uint8_t> y) {
+   std::vector<uint8_t> public_key_bytes;
+   public_key_bytes.reserve(x.size() + y.size() + 1);
+   public_key_bytes.push_back(0x04);  // means: uncompressed point encoding
+   public_key_bytes.insert(public_key_bytes.end(), x.begin(), x.end());
+   public_key_bytes.insert(public_key_bytes.end(), y.begin(), y.end());
+   return public_key_bytes;
+}
+
+int main() {
+   const std::string curve_name = "secp256r1";
+   const auto public_x_bytes = Botan::hex_decode("278309D4A88ADF89CA0E5328D3B655CF8949F2D9F9B2308AA22FE28202A315EC");
+   const auto public_y_bytes = Botan::hex_decode("AC457F18D1F3675D46E98ED2E509EE47AC2CB9A012F73263B30CD7248AEA6020");
+
+   const auto domain = Botan::EC_Group::from_name(curve_name);
+   const auto encoded_public_point = to_uncompress_point_encoding(public_x_bytes, public_y_bytes);
+
+   // This loads the public point into an ECDSA_PublicKey. Creating an
+   // ECDH_PublicKey would work the same way.
+   const auto public_key = Botan::ECDSA_PublicKey(domain, domain.OS2ECP(encoded_public_point));
+
+   std::cout << "Public Key (PEM):\n\n" << Botan::X509::PEM_encode(public_key) << '\n';
+}