feat: Added .pyi file for certificate.c

* Card ID: CCT-2233
* Added src/rhsm/_certificate.pyi interface file for Python
  module that is written in C language (certificate.c)
* This help could help IDEs with type-hinting

Signed-off-by: Jiri Hnidek <jhnidek@redhat.com>

Author: jirihnidek

src/rhsm/_certificate.pyi

@@ -0,0 +1,21 @@
+# Stubs interface file for rhsm._certificate CPython module written in C. The C code is
+# available at ./src/certificate.c. If you are editing the ./src/certificate.c file, you
+# should also edit this file to be able to provide better type hinting in IDEs.
+
+# TODO: Extend stubs of classes to provide better type hinting.
+
+class X509: ...
+class PrivateKey: ...
+class OpenSSLCertificateLoadingError: ...
+
+def get_public_key_algorithms(*args, **kwargs) -> list[str]:
+    """return a list of public key algorithms"""
+
+def get_signature_algorithms(*args, **kwargs) -> list[str]:
+    """return a list of signature algorithms"""
+
+def load(*args, **kwargs) -> X509:
+    """load a certificate from a file"""
+
+def load_private_key(*args, **kwargs) -> PrivateKey:
+    """load a private key from a file"""