Unify Celeritas problem setup and construction: `inp` namespace

[sethrj]

Creating a single setup mechanism for Celeritas, rather than the combination of ad-hoc runners that has developed over the last few years, will allow us to:

• Massively reduce duplicate code
• Create and destroy states in parallel so we can do gather/scatter operations (#1204)
• More tightly control how the parts of the code work to work together, so we don't have to (e.g.) assume users can add actions anytime before creating a state
• Better expose physics extension hooks to users
• Generate input definitions for front ends (#1204)

[sethrj]

See #1562, #1601, #1632, #1735, #1739, #1862, #1953, #1976