PHPLIB-1183: Initial performance tests

[alcaeus]

PHPLIB-1183

This PR adds initial performance tests. For the time being, this tests the performance of MongoDB\BSON\Document and MongoDB\BSON\PackedArray, and runs comparative benchmarks as defined in the BSON benchmark spec.

The performance tests for the BSON classes could theoretically be moved to the PHPC repo, but would require setting up an entirely new composer project. For the time being the tests can live here, but they should be moved to PHPC when we start regression testing so that any performance regressions can be caught directly while doing the work.

[jmikola]

Is this file missing a copyright block?

Do we actually need those for benchmarks? We don't use it in our test files.

[alcaeus]

We don't use them for tests, so I omitted them here as well.

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Edit: I noted other benchmark files included them. I removed them everywhere, as strictly speaking the copyright is already noted in the repository anyways.

[GromNaN]

LGTM.
I see the report is in the CI artifacts.
But there is no automatic comparison with previous build ATM?

[alcaeus]

But there is no automatic comparison with previous build ATM?

Not yet - there are a number of variables to control before making that viable, most notably ensuring that unrelated changes (e.g. newer server versions, operating systems, etc.) don't affect results. We'll also have to define metrics and add assertions to then make the build fail.