Meta Issue: Shipper configuration and I/O

[fearful-symmetry]

This is a meta-issue for tracking changes, bugs, or discussions related to the shipper's config and how it communicates with other components:

• Namespace config values in fleet output elastic-agent#1729
• Input configs from units should hard-code shipper connection settings elastic-agent#1744
• Standardize the shipper's gRPC TLS setup process #224
• Look into changing how the shipper recieves gRPC connection config #225