Implement PKCS12 decoder support

[pavlo-v-chernykh]

Extend the library to support decoding PKCS#12 container files, matching the decoding capabilities of the Java Keystore implementation.

• Implement functions to read and parse .p12/.pfx files.
• Support password-protected PKCS#12 files, including handling incorrect passwords gracefully.
• Ability to extract private keys from the PKCS#12 container.
• Extract public certificates and certificate chains.
• Retrieve and manage keys and certificates using aliases (friendly names), similar to Java Keystore.
• Support PKCS#12 files containing multiple key and certificate entries.
• Correctly parse and associate certificate chains with their respective private keys.
• Validate that PKCS#12 files generated by Java Keystore and OpenSSL can be correctly decoded by the library.
• Develop unit tests covering all decoding functionalities
• Perform tests with PKCS#12 files from Java Keystore and OpenSSL to verify interoperability.

Acceptance criteria

• Ability to read and parse password-protected PKCS#12 files.
• Successfully extract private keys, certificates, and certificate chains.
• Support for aliases and multiple entries within a PKCS#12 file.
• Successfully import PKCS#12 files generated by Java Keystore and OpenSSL.
• Unit tests cover at least 60% of the new decoding code.
• Interoperability tests with PKCS#12 files from other tools are documented and included.
• Code adheres to the project’s coding standards and guidelines.

Notes:

• Be cautious of third-party libraries to avoid license incompatibilities.
• Design with future maintenance and potential extensions in mind.
• Keep tests consistent across the codebase (stdlib or testify).
• Squash all commits into one in your PR.
• Rebase instead of merge
• Use the imperative mood in the first line of a commit message (e.g., "Implement PKCS12 decoder")
• Start the first line of the commit message from the capital letter