This PR addresses the request to modernize the Abseil dependency management by replacing the custom Bazel extension with a standard bazel_dep and eliminating the need for source patching.

Problem

The current setup used a custom Bazel extension (extensions.bzl) to download Abseil via http_archive and apply a source patch (abseil.patch) to modify absl/base/options.h. The patch forced Abseil to not use C++ standard library types:

-#define ABSL_OPTION_USE_STD_OPTIONAL 2
+#define ABSL_OPTION_USE_STD_OPTIONAL 0

This approach had several downsides:

• Required maintaining a custom extension
• Needed source patching which is fragile and non-standard
• Used an older Abseil commit instead of tagged releases

Solution

1. Replaced extension with direct bazel_dep

# Before: Custom extension
non_module_dependencies = use_extension("//:extensions.bzl", "non_module_dependencies")
use_repo(non_module_dependencies, "com_google_absl")

# After: Standard bazel_dep
bazel_dep(
    name = "abseil-cpp",
    version = "20230125.3",
    repo_name = "com_google_absl",
)

2. Replaced patch with compile-time configuration

Instead of patching source files, the same behavior is achieved through C++ preprocessor defines in .bazelrc.absl:

--cxxopt='-DABSL_OPTION_USE_STD_OPTIONAL=0'
--cxxopt='-DABSL_OPTION_USE_STD_STRING_VIEW=0' 
--cxxopt='-DABSL_OPTION_USE_STD_VARIANT=0'

These defines are applied for all platforms (Linux, macOS, Windows) alongside the existing DD_USE_ABSEIL_FOR_ENVOY macro.