[On-Cain] Add missing on-chain indices

[bryanchriswhite]

Objective

Resolve all outstanding comments like this:

// TODO_MAINNET: Use an index to iterate over the applications that have initiated
// the transfer action instead of iterating over all of them.

• x/supplier/keeper/unbond_suppliers.go:
  // TODO_IMPROVE: Use an index to iterate over suppliers that have initiated the ...
• x/supplier/keeper/supplier.go:
  // TODO_OPTIMIZE: Index suppliers by service ID ...
• x/application/keeper/transfer_applications.go:
  // TODO_MAINNET(@bryanchriswhite): Use an index to iterate over the applications that have initiated ...
• x/application/keeper/unbond_applications.go:
  // TODO_IMPROVE: Use an index to iterate over the applications that have initiated ...
• x/application/keeper/auto_undelegate.go:
  // TODO_IMPROVE: Once delegating applications are indexed by gateway address, ...

Origin Document

See #789 (comment)

Goals

• Optimize on-chain lookups, particularly in end-blocker loops.

Deliverables

• One or more PR(s) which add the desired indices (and usage thereof) to the supplier module.
• One or more PR(s) which add the desired indices (and usage thereof) to the application module.

Non-goals / Non-deliverables

• Refactoring unbonding/unstaking logic beyond the scope of this issue's goals.

General deliverables

• Comments: Add/update TODOs and comments alongside the source code so it is easier to follow.
• Testing: Add new tests (unit and/or E2E) to the test suite.
• Makefile: Add new targets to the Makefile to make the new functionality easier to use.
• Documentation: Update architectural or development READMEs; use mermaid diagrams where appropriate.

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Creator: @bryanchriswhite
Co-Owners: