Dynamic fee setter (WeightedPool)

[jubeira]

Description

Add the capability to modify the swap fee from certain callers.
Applicable for weighted pool for now, whenever the feature is allowed at deployment time.

Pending:

• Tests.
• Setting solver / changing solver

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

N/A