fix: headers being incorrectly matched in the gRPC metadata

[kushthedude]

Signed-off-by: kushthedude kushthedude@gmail.com
fixes #255

Context around additional changes:

• Aggregated gRPC helper functions into one grpc_helper file.
• Made few internal methods private as they won't be needed anywhere else(like grpc_middleware handling).
• Few naming conventions changes according to best practices