Fixes #1859 [EXPORTER] Boolean environment variables not parsed per the spec

Changes in SDK

• Moved the implementation of GetEnvironmentVariable() from .h
  to GetRawEnvironmentVariable() in .cc

• Changed the prototype to return bool exists,
  to decouple environment variables existence from empty value.

• Implemented typed helpers to properly parse values:

  • GetBoolEnvironmentVariable(),
  • GetDurationEnvironmentVariable(),
  • GetStringEnvironmentVariable()

Changes in Exporters environment variables

• Moved GetXXXDefaultYYY() helpers from otlp_environment.h to .cc

• Renamed all helpers for orthogonality:

  • helpers dedicated to a signal have the signal name, in plural form
  • helpers dedicated to Http or Grpc are named accordingly.

• This exposed many bugs and inconsistencies, for example for Endpoint()

  • Before:

    • GetOtlpDefaultGrpcEndpoint() -- Misnamed, it is for traces
    • GetOtlpDefaultHttpEndpoint() -- Misnamed, it is for traces
    • GetOtlpDefaultHttpLogEndpoint() -- GrpcLog missing
    • GetOtlpDefaultMetricsEndpoint() -- is it for Http or Grpc ?

  • After:

    • GetOtlpDefaultGrpcTracesEndpoint()
    • GetOtlpDefaultGrpcMetricsEndpoint()
    • GetOtlpDefaultGrpcLogsEndpoint()
    • GetOtlpDefaultHttpTracesEndpoint()
    • GetOtlpDefaultHttpMetricsEndpoint()
    • GetOtlpDefaultHttpLogsEndpoint()

• This renaming is a potential breaking change for logs (now in plural form),
  which is acceptable because logs are not fully implemented yet.

• This renaming is a potential breaking change if a user application
  used existing helpers for traces and metrics,
  so original names are also provided for backward compatibility.
  For example:

// Compatibility with OTELCPP 1.8.2
inline std::string GetOtlpDefaultHttpEndpoint()
{
  return GetOtlpDefaultHttpTracesEndpoint();
}

• Added all the helpers required to parse environment variables
  defined in the spec, for example for traces SSL/TLS options:

  • GetOtlpDefaultTracesSslCertificatePath();
  • GetOtlpDefaultTracesSslCertificateString();
  • GetOtlpDefaultTracesSslClientKeyPath();
  • GetOtlpDefaultTracesSslClientKeyString();
  • GetOtlpDefaultTracesSslClientCertificatePath();
  • GetOtlpDefaultTracesSslClientCertificateString();

• In otlp_environment.cc, implemented typed helpers
  for environment variables that can have dual names:

  • GetBoolDualEnvVar()
  • GetDurationDualEnvVar()
  • GetStringDualEnvVar()

• Refactored all the GetXXXDefaultYYY() implementation to use
  these helpers.

• For special cases like GetOtlpDefaultTracesIsInsecure(),
  provided a migration path from old variables (OTEL_EXPORTER_OTLP_TRACES_SSL_ENABLE)
  to new variables (OTEL_EXPORTER_OTLP_TRACES_INSECURE)

• Prepared the code for future variables deprecation,
  with #define WARN_DEPRECATED_ENV.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed