[services] Tunnelbroker - Add DYNAMODB_MAX_BATCH_ITEMS to Constants

Summary:
This diff is a part of the stack that introduces batch operations with the DynamoDB.

The DYNAMODB_MAX_BATCH_ITEMS constant represents the maximum allowable batch size for DynamoDB which is 25 items.

Linear task: ENG-1158

Test Plan: No testing is needed, it's just a constant.

Reviewers: karol-bisztyga, palys-swm

Reviewed By: karol-bisztyga, palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phab.comm.dev/D4372