PHPLIB-748: Support 'let' option for multiple CRUD commands

[aleksandr-rudo]

https://jira.mongodb.org/browse/PHPLIB-748

This is a draft PR till the ticket https://jira.mongodb.org/browse/DRIVERS-2270 will be resolved.

[jmikola]

@aleksandr-rudo: Feel free to sync the CRUD spec tests with mongodb/specifications#1174. You can temporarily write Synced with mongodb/specifications#1174 in the commit message for now and we can update that to the final commit once that works is merged.

I don't think there will be any spec test changes, so the only further work for you should be amending the commit message. In the meantime, pulling in the new JSON files should allow us to pass CI here, and let this PR server as a proof of concept (POC).

[jmikola]

@aleksandr-rudo: I forgot to mention this earlier but we'll also need to document the new option.

You can review commits for /docs for examples of how this was done for previous PRs. The most relevant, recent commit may be when let was added for aggregate() in a4b172c.

The docs are built with separate tooling, which is briefly discussed in CONTRIBUTING.md. You can try to get that running locally, but if you run into trouble I'm happy to just test the changes you make and fix any mistakes in the interest of time. The docs themselves are RST files with separate includes for method parameters and options, and the method-specific include files can inherit common includes (viewing previous commits where options were documented should help explain how everything is organized).

[jmikola]

mongodb/specifications#1174 is merged so feel free to update the sync commit to point to mongodb/specifications@51ca03b

[jmikola]

#913 was just merged and pulls in the same tests, since the sync commit was more recent. You can either rebase this branch on master or merge master into your branch (in which case the merge commit produced will disappear when we finally merge this PR).

I also added skips for existing spec tests in that PR, so I expect you will just need to remove the skips I added for the let tests.

[jmikola]

@aleksandr-rudo: I just took a look through each file again. Let me know when this is ready for a final review by flipping it from draft status and I'll check it out locally and build the docs to confirm all is well there.

[jmikola]

Docs build locally. LGTM.

I opened PHPLIB-842 to track the intermittent change stream spec tests failures.