Switch to pixi and rattler-build

[lohedges]

This PR switches to using pixi for environment creation and management and rattler-build for building conda packages.

Additional updates include:

• Correctly handle the different input formats for the perturbed_system option. When a user passes in a system object directly, e.g. when scripting somd2 rather than using the CLI, we now stream the system to the output directory and store the path to the file in the YAML config file. This allows the system to be reconstructed correctly when running from a restart.
• Custom lambda schedules and restraints are now serialised to the YAML file using a hexadecimal string representation of the object, allowing it to be reconstructed on load. This fixes restart issues for users running somd2 via scripts with custom schedules. As we formalise robust schedules, these can be added as supported keywords, which will make the config more readable.