[agent] Move the certificates generation/loading to a module

[ansasaki]

Description:

During startup, the agent creates or loads the private key and mTLS certificate, and loads the trusted certificates.
The logic is implemented as part of the main() function.
This is about refactoring the code and moving this initialization to a module in the keylime library.
This way the code can be easily reused, for example in case the push and pull model are implemented in separate binaries.

• Enhancement: 103: Agent-driven attestation
• Project board: https://github.com/orgs/keylime/projects/5

Checklist:

• Changes implemented
• Tests added
• Documentation updated
• If configuration options are modified, the templates should be updated