[INLONG-10115][Agent] Change the offset save format from long to string

[justinwwhuang]

Fixes #10115

Motivation

Offset needs to be changed to save in string format because some data sources have non integer positions

Modifications

Change the offset save format from long to string

Verifying this change

(Please pick either of the following options)

• [x] This change is a trivial rework/code cleanup without any test coverage.

• [ ] This change is already covered by existing tests, such as: (please describe tests)

• [ ] This change added tests and can be verified as follows:

  (example:)

  • Added integration tests for end-to-end deployment with large payloads (10MB)
  • Extended integration test for recovery after broker failure

Documentation

No doc needed