Centralize shared OpenTelemetry code into source/extensions/common/opentelemetry/

[mmorel-35]

Centralizes shared OpenTelemetry types, utilities, and exporters into source/extensions/common/opentelemetry/, aligning the directory structure and naming conventions with opentelemetry-cpp.

Changes Made

SDK structure (mirrors opentelemetry-cpp)

• Added sdk/common/types.h with a shared AttributeValue variant type (Sdk::Common namespace) and #include <cstdint>
• Added sdk/trace/constants.h, sdk/logs/constants.h, and sdk/metrics/constants.h with per-signal OTLP service method and HTTP endpoint constants (Sdk::Trace, Sdk::Logs, Sdk::Metrics namespaces) — avoids empty placeholder files and provides real utility, mirroring the approach in Centralize OpenTelemetry Constants and Helpers in source/extensions/common/opentelemetry #41011

Exporters

• Moved OTLP trace exporter (gRPC + HTTP) to exporters/otlp/ under Exporters::Otlp namespace; original tracer headers retained as forwarding/compat headers
• Added exporters/otlp/environment.h/cc with Exporters::Otlp::GetUserAgent() — mirrors GetOtlpDefaultUserAgent() from opentelemetry-cpp's otlp_environment.h; used by both gRPC and HTTP exporters as it is an exporter identity concern, not HTTP-specific

Attribute utilities (mirrors opentelemetry-cpp)

• Moved populate_attribute_utils.h/cc to exporters/otlp/ under Exporters::Otlp namespace — mirrors OtlpPopulateAttributeUtils from opentelemetry-cpp, which lives alongside the OTLP exporters
• Renamed getStringKeyValue() → makeKeyValue()
• otlp_utils.h / otlp_utils_lib kept as backward-compatibility forwarding shims with using OtlpUtils = PopulateAttributeUtils
• Fixed populateAnyValue() to handle all 17 AttributeValue alternatives, including array types for bool/int/uint/int64/double/uint64/bytes (previously fell into IS_ENVOY_BUG); uint64 values exceeding INT64_MAX are now clamped rather than silently wrapping to negative

Cross-extension dependency fix

• stat_sinks/open_telemetry/open_telemetry_http_impl.cc now depends on common/opentelemetry directly instead of access_loggers/open_telemetry
• Updated grpc_trace_exporter.cc, grpc_access_log_impl.cc, and open_telemetry_impl.cc to consume the new sdk/{trace,logs,metrics}/constants.h rather than inline string literals

Type safety & correctness

• Added resource_spans_size() / scope_spans_size() guards in logExportedSpans() to prevent out-of-bounds proto access
• OtelAttributes changed from std::map to absl::flat_hash_map; compile-time static_assert added to test

Tests & docs

• Moved populate_attribute_utils_test.cc to test/extensions/common/opentelemetry/exporters/otlp/ to mirror source layout
• Split tests into ExporterEnvironmentTest + PopulateAttributeUtilsTest; renamed test cases to match new API names (MakeKeyValue, GetUserAgent)
• Added #include <type_traits>, #include "absl/strings/match.h" to tests
• Updated README.md to document new API names and locations (PopulateAttributeUtils in exporters/otlp/, Exporters::Otlp::GetUserAgent(), environment_lib, and the new sdk/{trace,logs,metrics}_constants_lib targets)
• Updated changelog

[mmorel-35]

/cc @kyessenov

It's based on #41011 , hopefully the comments you've already provided are addressed there.