Issue:
refactor: move config defaults from TOML files to `Default` trait

## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [x] Refactoring
- [ ] Dependency updates

## Description
<!-- Describe your changes in detail -->
This PR moves default values for `router` and `drainer` binaries' configuration from the `defaults.toml` files to the implementation of the `Default` trait. Since we add a `#[serde(default)]` to the `Settings` struct, it will use the `Default` trait implementation to provide the default values.

## Motivation and Context
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If it fixes an open issue, please link to the issue here.

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This should help catch errors in the default configuration values during compilation, and help reduce binary size.

## How did you test it?
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Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
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Ran the `router` and `drainer` binaries, verified that they both work with the current defaults provided.

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [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](/CHANGELOG.md) entry if applicable


Resolution:
<think>
PR Context:
- Title: chore(errors): cleanup unused `ApplicationError` variants
- Merged: 2023-01-12T11:06:31Z
- Files changed: 2 files
- Commits: 3 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [x] Refactoring
- [ ] Dependency updates

## Description
<!-- Describe your changes in detail -->
This PR removes some variants of the `ApplicationError` enum that are no longer used. Found that there were quite a lot of unused variants when working on #352.

## Motivation and Context
<!--
Why is this change required? What problem does it solve?
If it fixes an open is
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** chore(errors): cleanup unused `ApplicationError` variants (#356)
Files to modify:


**crates/router/src/core/errors.rs**
  Modify existing code

**crates/router/src/types/api.rs**
  Modify existing code