Enhance documentation to specify processing of Settlement Model Handler

[ggrg]

Goal:

As an OSS codebase contributor
I want to update documentation in relation to the proposed Settlement Model Handler
so that it could be reviewed by the DA and later used as reference during implementation

Tasks:

• Update DB schema diagram [@ggrg]
• Collaborate on the update of the OSS Settlements FSD PR [@ggrg]
• Add new sequence digram to describe processing for SettlementModelHandler [@ggrg]
• Update existing diagrams
  • Overall transfer fulfil [@ggrg]
  • Fulfil Handler [@ggrg]
  • Settlement Model Handler [@ggrg]
  • closeSettlementWindow [@ggrg]
  • createSettlement [@ggrg]
  • updateSettlementById [@ggrg]
• Propose changes to the DA
• Update designs with DA requested changes, if any

Acceptance Criteria:

• Designs are up-to date
• All CI/CD checks pass
• Designs are presented and approved by the DA
• Designs are presented to the team

Pull Requests:

• feature/1218 Settlement Model Handler Design central-ledger#607 <-- pushed as mojaloop feature branch
• feature/1218 Settlement Model Handler Design documentation#176

Follow-up:

• Implement Settlement Model Handler to central-ledger #1217

Dependencies:

• PR #173 by @mjbrichards

Accountability:

• Owner: @ggrg
• QA/Review: @ggrg, @MichaelJBRichards