add daemonization

Add flags to rez-env and to suite wrappers to daemonize a command.

This will involve adding a new tool, rez-daemonize-wrapper. This will be the process actually daemonized, and it will subproc the target command. This is needed because otherwise Rez will clean up the context file before the child process terminates, meaning that any attempt in the child proc to use the rez API and query the current context would fail. rez-daemonize-wrapper would just run the child proc and then cleanup the rez tempfiles on exit.

I expect to add these args:

rez-env --daemon

my_suite_tool ++daemon

Start with the rez-env entry point, the suite wrappers, and the proposed rez-daemonize-wrapper tool. Trace how the command context and temporary files are handled when a child process exits. Done means rez-env --daemon and my_suite_tool ++daemon launch through the wrapper and preserve the context until the child process terminates, then clean up temporary files.

Written by the indexing model from the issue text.