Update OtlpGrpcClientOptions to populate options for gRPC client sharing

Problem:

OtlpGrpcClientOptions has only a = default constructor and it initializes all options to null or c++ default values. The exporter specific options classes read environment variables and set the otel-spec defined defaults. This makes it challenging to construct a standalone OtlpGrpcClient with spec-compliant defaults when sharing the client among multiple exporters.

Desired use case:

Construct a gRPC client and share it with all three signal exporters.

OtlpGrpcClientOptions client_opts;  // this should read env vars or set spec default values
auto shared_client = OtlpGrpcClientFactory::Create(client_opts);

// use a new constructor of signal options classes copy from the shared client options
// signal specific constructors override options that are not fixed at client creation
OtlpGrpcExporterOptions          trace_opts(client_opts);         
OtlpGrpcMetricExporterOptions    metric_opts(client_opts);  
OtlpGrpcLogRecordExporterOptions log_opts(client_opts);   

auto trace_exporter  = OtlpGrpcExporterFactory::Create(trace_opts, shared_client);
auto metric_exporter = OtlpGrpcMetricExporterFactory::Create(metric_opts, shared_client);
auto log_exporter    = OtlpGrpcLogRecordExporterFactory::Create(log_opts, shared_client);

Proposed change:

• Add helpers in otlp_environment.h/cc, that read the OTLP signal independent environment variables (OTEL_EXPORTER_OTLP_ENDPOINT, OTEL_EXPORTER_OTLP_TIMEOUT, ...) and fall back to spec defaults (endpoint=http://localhost:4317, ...) for all options needed by the grpc client.

• Add a default constructor to OtlpGrpcClientOptions in a .cc file that uses those generic helpers to populate the options

• Add a void* constructor to OtlpGrpcClientOptions that skips populating the options for use with the void* constructors of signal specific options classes.

• Add constructors for each OtlpGrpcClientOptions derived class that takes a OtlpGrpcClientOptions instance and copies options fixed on client creation then overrides signal-specific options as required. See the table below for which options may be overridden when a shared client is used.

Start with otlp_environment.h/cc and the OtlpGrpcClientOptions and signal-specific option classes described in the issue. Trace how current exporter options read environment variables, then implement and verify the shared-client constructors and option-copying behavior against the override table, including the listed OTLP defaults.