[1/n][sl][github][gh stacks] support headers and list params

[raydatray]

ctx

github added native stacked pull requests, it would be nice to add native support to it for sapling!

this stack lets sl pr submit and sl pull via a new github.pr.workflow = stacked mode

changes made

• github_gh_cli.make_request accepts custom headers (-H, needed for the X-GitHub-Api-Version preview header)
• _format_param supports list values using the gh api repeated-field syntax (pull_requests[]=101)
• mock_utils updated to match

test plan

doctests in _format_param (registered in test-doctest.py), existing github .t suite

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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]

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