Adds an interface to PDLP, using a similar structure to the GLOP interface.

Also switch the Glop interface to use a proto struct rather than a text string for parameters. My impression is that most users won't be familiar with the text format for protos and that it's easier to work with the normal proto object directly.

Notes:

• We were not able to polish off a numpy interface thanks to pybind11_protobuf being incompatible with cmake. We'll have something for the next release. In the meantime, there will be extra overhead in translating the model between python and C++.
• The apply function is duplicated between the GLOP and the PDLP interface. This code is going to change (and likely diverge) when we have numpy interfaces, so it seemed premature to refactor to share the code.
• I'm not sure of the right way to check that OR-Tools is version 9.3 or better. (Otherwise the code will error out.)
• There's a internal warning printed after each solve, this will go away in the version bump of OR-Tools: String field 'operations_research.MPSolutionResponse.solver_specific_info' contains invalid UTF-8 data when parsing a protocol buffer. Use the 'bytes' type if you intend to send raw bytes.

Closes #1649

CC @dapplegate @lperron @Mizux