instance should be passed into QiskitFunctionsCatalog

[pandasa123]

Description

This is an interesting issue, where we need to have a discussion. Currently, instance is passed in directly to a Qiskit Function, and is managed function-by-function. This is inconsistent with how instance is handled with Runtime, where QiskitRuntimeService consumes instance as an optional kwarg when instantiating the object

This opens the floor to an interesting question: beyond setting this an env_var for Function Providers, should QiskitFunctionsCatalog do anything in its underlying behaviours (authentication, list(), jobs(), etc)? (possibly out of scope for the first pass)

Acceptance Criteria

• I should be able to pass in instance as an optional kwarg into QiskitFunctionsCatalog
• If I pass in instance, functions (IBM + 3rd-party functions) should be able to access this with QISKIT_IBM_INSTANCE as an environment variable