[3/n][sl][github][gh stacks] add stacked workflow for submit

[raydatray]

ctx

introduces the new native github stack workflow

changes made

• new SubmitWorkflow.STACKED variant, selected via github.pr-workflow=stacked
• chains each PR's base to the head branch of the PR below it (like single), factored into SubmitWorkflow.uses_chained_bases()

test plan

new test-ext-github-pr-submit-stacked.t

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Stack created with Sapling. Best reviewed with ReviewStack.

• [7/7][sl][github][gh stacks] update some documentation #1398
• [6/n][sl][github][gh stacks] wire up gh stacks to sl pull #1397
• [5/n][sl][github][gh stacks] wire up gh stacks to sl pr submit #1396
• [4/n][sl][github][gh stacks] update submit body for stacks #1395
• -> [3/n][sl][github][gh stacks] add stacked workflow for submit #1394
• [2/n][sl][github][gh stacks] add support for stack endpoint #1393
• [1/n][sl][github][gh stacks] support headers and list params #1392

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D114402675. (Because this pull request was imported automatically, there will not be any future comments.)