feat: Add OCI Functions invoke client support (FunctionsInvokeClient)

[paezraphael]

Summary

Adds support for OCI Functions (InvokeFunction operation) to allow PHP applications to invoke OCI Functions directly with native OCI HTTP request signing, without having to shell out to the OCI CLI or manually implement signing logic.

Description & Key Features

• src/Oracle/Oci/Functions/FunctionsInvokeClient.php: Client implementing synchronous invokeFunction and invokeFunction_Helper, structured like ObjectStorageClient with explicit per-function endpoint support.
• src/Oracle/Oci/Functions/FunctionsInvokeAsyncClient.php: Asynchronous version supporting invokeFunctionAsync and invokeFunctionAsync_Helper using Guzzle promise interface.
• examples/FunctionsInvokeExample.php: Usage example demonstrating function invocation.
• tests/Functions/FunctionsInvokeClientTest.php: Unit test suite covering endpoint construction, signature generation, header validation, request body payload, and exception handling.

Compatibility & Dependencies

• Compatible with php >= 5.6 and guzzlehttp/guzzle ^6.5.
• No new external dependencies.
• Commits signed off (Signed-off-by: Raphael Paez <paezraphael@gmail.com>).

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[paezraphael]

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