*: label external the request source via sessionvar

[nolouch]

What problem does this PR solve?

Issue Number: ref #44769

Problem Summary:

currently, we cannot know requests from external tools, like using the lightning-cli with tidb backend model to import data.

What is changed and how it works?

add a sessionvar to complement the RequestSourceType.
- external tools need start a session with setting the variable, then label the source origin

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[glorv]

/cc @bb7133 @BornChanger

[ti-chi-bot]

@glorv: Your lgtm message is repeated, so it is ignored.

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2023-07-07 08:55:43.530405399 +0000 UTC m=+63035.300744111: ☑️ agreed by glorv.
• 2023-07-10 09:48:00.360071065 +0000 UTC m=+325372.130409778: ☑️ agreed by crazycs520.

[ti-chi-bot]

@nolouch: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-br-integration-test	760984b486d842ab84494b9411b5a1f0fc384b70	link	true	/test pull-br-integration-test

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[nolouch]

/retest-required

[nolouch]

ptal @bb7133

[hawkingrei]

/hold

[hawkingrei]

/unhold

[hawkingrei]

/hold

[hawkingrei]

/unhold

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: crazycs520, glorv, HuSharp, qw4990

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