Auto merge of #11593 - jdm:netmon, r=nox

Make the net monitor panel in FF's devtools show meaningful output.

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1. Advertise support for the network monitor in the initial protocol communication.
1. Only notify the developer tools server about the final request in an HTTP transaction.
1. Add timing information for connecting to the HTTP server and sending the HTTP request.
1. Reduce duplication between various networkEventUpdate structures by creating a helper function
that merges two JSON structures together. This also corrects the JSON structure so the devtools
client interprets the output correctly.
1. Calculate various header size fields correctly.
1. Remove unnecessary usize->u32 casts by making the appropriate fields usize.
1. Add header values to request and response header messages.
1. Support triggering page reloads via the devtools client.

I apologize that these aren't broken apart. I was making a lot of changes trying to figure out why the panel wasn't working right, and a lot of them were tangled together.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes do not require tests because there are no automated tests for the devtools server.

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