[4/n][sl][github][gh stacks] update submit body for stacks

[raydatray]

ctx

github renders native stacks in the PR UI itself so the sapling footer would be redundant - lets remove it when submitting via native stacks

changes made

• create_pull_request_title_and_body takes a stack_list flag; the stacked workflow omits the footer, other workflows are unchanged

test plan

updated mocks + test-ext-github-pr-submit-stacked.t

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 D114402698. (Because this pull request was imported automatically, there will not be any future comments.)