Issue:
[REFACTOR] Update KafkaStore to hold variables for tenant ID

Update the [`KafkaStore`](https://github.com/juspay/hyperswitch/blob/c20ecb855aa3c4b3ce1798dcc19910fb38345b46/crates/router/src/db/kafka_store.rs#L75-L79) to hold a variable for tenant-ID.

This can be achieved by simply adding a new field in the `KafkaStore` struct.
Update the KafkaStore constructor method to accept a Tenant-ID.

Tenant-ID can be a new type field backed by a string;

```rust
pub struct TenantID(String);
```

For now the KafkaStore can be initialized with a fixed tenantID `"default"`

Resolution:
<think>
PR Context:
- Title: refactor(db): Add TenantId field to the KafkaStore struct
- Merged: 2024-05-08T13:19:57Z
- Files changed: 3 files
- Commits: 7 commits

Implementation approach:
## Type of Change
<!-- Put an `x` in the boxes that apply -->
Add TenantId field to the KafkaStore struct
#4496 

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [X] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
- Add TenantId field to the KafkaStore struct
- Add the tenant_id parameter to the constructor of Kafkastore 


### Additional Changes

- [ ] This PR modifies the API contract
- [ ] T
</think>

To implement this, you'll need to modify:


**Detailed Implementation:**


**Commit 1:** refactor(db): Add TenantId field to the KafkaStore struct (#4512)
Files to modify:


**crates/router/src/db.rs**
  Remove:
    - mod: private::kafka_store
  Add:
    - mod: pub::kafka_store

**crates/router/src/db/kafka_store.rs**
  Add:
    - struct: pub::TenantID

**crates/router/src/routes/app.rs**
  Modify existing code