TLS termination for incoming client connection

[mostafa]

Ticket(s)

• client: TLS termination of incoming connections from database clients #361

Description

This PR adds TLS termination for incoming Postgres clients, which helps enable the following scenarios:

1. Client disables SSL:

%%{init: {'theme': 'base'} }%%
flowchart LR
    A(Client) <-->|plaintext| B(GatewayD)
    B <-->|plaintext| C(DBMS)

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2. Client prefers SSL:

%%{init: {'theme': 'base'} }%%
flowchart LR
    A(Client) <-->|plaintext if TLS disabled, encrypted otherwise| B(GatewayD)
    B <-->|plaintext| C(DBMS)

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3. Client requires SSL:

%%{init: {'theme': 'base'} }%%
flowchart LR
    A(Client) <-->|encrypted if TLS enabled, rejected otherwise| B(GatewayD)
    B <-->|plaintext| C(DBMS)

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Related PRs

• TLS termination and its config docs#32

Development Checklist

• I have added a descriptive title to this PR.
• I have squashed related commits together.
• I have rebased my branch on top of the latest main branch.
• I have performed a self-review of my own code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have added docstring(s) and type annotations to my code.
• I have made corresponding changes to the documentation (docs).
• I have added tests for my changes.

Legal Checklist

• I have read the CONTRIBUTING document.
• I have read and understood the Code of Conduct.
• I have read and agreed to the Apache CLA (required).
• I have read and agreed to the LICENSE (required).