Bringing all default options to one place

[nirandaperera]

[C++/Python] Centralize option keys and defaults via OptionDescriptor

Option keys and default values were scattered as raw string literals across C++ and Python call sites, making it easy for the two languages to drift and for typos to go unnoticed. This PR consolidates every recognised configuration option into a single source of truth in cpp/include/rapidsmpf/config.hpp and exposes the same keys and defaults to Python through a new rapidsmpf.config_defaults module that pulls the values directly from the C++ header at import time.

• Add OptionDescriptor{key, default_val} (non-templated) in config.hpp, with both fields stored as std::string_view. Defaults are written as string literals (e.g. "false", "16", "1ms", "WARN") since options are always parsed from their string representation — the descriptor's default flows through the same factory the call site uses for user-supplied values.
• Replace ad-hoc key/default literals across statistics.cpp, buffer_resource.cpp, pinned_memory_resource.cpp, communicator.cpp, ucxx.cpp, coro_executor.cpp, and memory_reserve_or_wait.cpp with the descriptor-based API. Typed defaults (bool / size_t / uint32_t) are parsed via parse_string<T> so the canonical string form is the single source of truth.
• Widen Options::get to accept std::string_view so descriptor keys can be passed directly (existing std::string / const char* callers still work via implicit conversion).
• Widen parse_string<T> to accept std::string_view so descriptor defaults flow through unchanged.
• Introduce rapidsmpf/config_defaults.{pyx,pyi} exposing one Final[str] constant per option (e.g. BUFFER_RESOURCE_NUM_STREAMS) plus a read-only DEFAULTS: Mapping[str, str] (MappingProxyType) sourced from the C++ header via cdef extern. A single RMPF_OPT macro re-exposes key.data() and default_val.data() as null-terminated const char* aliases — safe because both views are initialised from string literals.
• Migrate Python consumers (memory/buffer_resource.pyx, streaming/core/memory_reserve_or_wait.pyx, integrations/core.py, tests/test_config.py) to import from config_defaults instead of using raw string keys / hard-coded defaults. Consumers that need typed values parse the string default on-site (e.g. int(_OPTION_DEFAULTS[BUFFER_RESOURCE_NUM_STREAMS]), parse_boolean(_OPTION_DEFAULTS[STREAMING_ALLOW_OVERBOOKING_BY_DEFAULT])), mirroring the C++ side.
• Promote the previously unnamed streaming option to streaming::AllowOverbookingByDefaultOption and drop the Factor suffix from the pinned-pool size descriptors.

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[madsbk]

Remove CMakeFiles/test_sources.dir/test_host_buffer.cpp.o

[madsbk]

Why not just keep std::string const& key?

[nirandaperera]

wanted to preserve the constexpr for OptionDescriptor

[madsbk]

Maybe just use std::string to avoid this complication, I don't think it will make any difference performance wise?

[madsbk]

Why not std::string const& text and avoid a copy?

[madsbk]

@nirandaperera could you reconsider the string-handling complexity here?

I would like to keep a plain std::string const& key in Options::get, and drop the StringHash / transparent-lookup machinery along with it.

Tracing it back, the whole apparatus seems to exist to serve one decision: making the OptionDescriptors inline constexpr. That forces std::string_view members, which then forces the heterogeneous-hashing dance in get() to avoid a temporary allocation per call.

But nothing in the codebase actually consumes the descriptors in a constexpr context. If we switch the descriptors to inline const with std::string members, the chain collapses:

options.get(EnabledOption.key, ...)

binds directly to std::string const&, with no temporary and no transparent hashing.

The only thing that matters at runtime here is the performance of Options::get() itself.