[improve][pip]PIP-296: support storing brokerClient certificates in metadata store

[ericsyh]

Fixes #xyz

Main Issue: #xyz

PIP: #xyz

Motivation

PIP for improving the createCluster API under configuring the geo-replication to a remote cluster with TLS enabled.

Modifications

Add the design doc for PIP 295. Pls note this PIP design doc is incomplete since I can't cover the detailed design and will engage some developing resources to help fulfill.

Verifying this change

• Make sure that the change passes the CI checks.

(Please pick either of the following options)

This change is a trivial rework / code cleanup without any test coverage.

(or)

This change is already covered by existing tests, such as (please describe tests).

(or)

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

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository:

[ericsyh]

@mattisonchao @poorbarcode PTAL.

[github-actions]

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