Allow passing asset managers to ComposableStablePool

[sajal]

Description

This PR allows the ability to pass asset managers when deploying ComposableStablePool. Previously the contract hardcoded zero-address.

Background info at #2141.

Type of change

• Bug fix
• New feature
• Breaking change
• Dependency changes
• Code refactor / cleanup
• Documentation or wording changes
• Other

Checklist:

• The diff is legible and has no extraneous changes
• Complex code has been commented, including external interfaces
• Tests are included for all code paths
• The base branch is either master, or there's a description of how to merge

Issue Resolution

Fixes #2141

[EndymionJkb]

Asset managers are extremely powerful: they can withdraw all funds. Allowing pools to be deployed from the canonical factory with arbitrary asset managers would greatly compromise security.

The safe way to deploy a pool with asset managers is illustrated by Linear and Managed Pools: make a factory that deploys both the pool and a specific asset manager contract (e.g., a LinearPoolRebalancer or pool controller). Then users would be able to review the contracts and determine the safety of pools deployed from that factory.