feat(esp-tls): Add unified private key interface via esp_key_config_t

Add ESP_KEY_SOURCE_BUFFER and ESP_KEY_SOURCE_PSA key sources so all
hardware backends (DS, ECDSA, secure element) are accessed via PSA
key IDs through a single esp_tls_cfg_t.client_key field.

(cherry picked from commit 36090b7161)

tools/mocks/esp-tls/CMakeLists.txt

@@ -8,6 +8,6 @@ idf_component_mock(INCLUDE_DIRS "${original_esp_tls_dir}"
                                 "${original_esp_tls_dir}/esp-tls-crypto"
                    MOCK_HEADER_FILES ${original_esp_tls_dir}/esp_tls.h
                                      ${original_esp_tls_dir}/esp-tls-crypto/esp_tls_crypto.h
-                    REQUIRES mbedtls
+                    REQUIRES mbedtls esp_security
                    )
 target_compile_options(${COMPONENT_LIB} PRIVATE -Wno-array-parameter)