feat(router): add generic merchant static routing config

[vspecky]

Type of Change

• [ ] Bugfix
• [x] New feature
• [ ] Enhancement
• [x] Refactoring
• [ ] Dependency updates

Description

This PR removes the legacy CustomRoutingRules type for describing static routing and makes the static routing configuration in the merchant account generic. It also adds a simple Single static routing algorithm which will allow the merchant to route all payments to a single connector and its implementation in payments core.

Additional Changes

• [x] This PR modifies the database schema
• [ ] This PR modifies application configuration/environment variables

• DB up.sql
• DB down.sql

Motivation and Context

This serves as scaffolding to integrate other forms of static routing into hyperswitch.

How did you test it?

Compiler-guided, stripe payments.

Checklist

• [x] I formatted the code cargo +nightly fmt --all
• [x] I addressed lints thrown by cargo clippy
• [x] I reviewed submitted code
• [ ] I added unit tests for my changes where possible
• [ ] I added a CHANGELOG entry if applicable