[services][blob] Provide S3 and DB to service, add more constants

Summary:

• Provided the S3 and DynamoDB clients to the MyBlobService implementation, the very similiar way as it is done in Identity Service
• Added some constants and grouped them. Their values are taken from various places across the C++ implementation.

Depends on D5681

Test Plan: The service is able to build and start as expected.

Reviewers: varun, jon, tomek, marcin

Reviewed By: varun, tomek

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5682