[REFACTOR] Update KafkaStore to hold variables for tenant ID

[lsampras]

Update the KafkaStore 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;

pub struct TenantID(String);

For now the KafkaStore can be initialized with a fixed tenantID "default"

[subhajit20]

hey @lsampras can You review my pr once?

[lsampras]

@subhajit20 Thanks for the PR. I'll assign you this issue