Implement (almost) all global options

[ambv]

This pull request is split into two commits, doing those things:

Commit 1: moves option handling from the Python C API to CFFI

This simplifies future edits and removes some static typing weirdness around the _pygit2.pyi file needing Python-level enums to express the C-level functional API.

Mostly, this moves pygit2 in the direction of being more CFFI, which will eventually lead to beautiful things like not needing to push new wheels every time a new version of Python comes out (i.e. #1046).

This diff is actually red (more lines removed than added).

Commit 2: implements all remaining options

...save for GIT_OPT_SET_ALLOCATOR, which I'm pretty confident is useless for Python users, and next to impossible to get right.

But other options that were missing so far are handled, including three network related that I was really looking forward to:

• SET_SERVER_TIMEOUT,
• SET_SERVER_CONNECT_TIMEOUT, and
• ENABLE_HTTP_EXPECT_CONTINUE.

All new options have a corresponding test.

[ambv]

A strarray is an array of strings, not a single string.

[ambv]

Otherwise this fails on macOS where mkdir doesn't use the GNU version.

[ambv]

I can reproduce the openssl segfault on Ubuntu with:

$ ci/cp310/bin/pytest test/test_options.py::test_add_ssl_x509_cert test/test_settings.py::test_ssl_methods

...

test/test_options.py::test_add_ssl_x509_cert PASSED
test/test_settings.py::test_ssl_methods Fatal Python error: Segmentation fault

Current thread 0x0000ffffa870f420 (most recent call first):
  File "/home/ubuntu/pygit2/pygit2/options.py", line 815 in option
  File "/home/ubuntu/pygit2/pygit2/settings.py", line 253 in add_ssl_x509_cert

...

So far I had a test with a dummy cert there, which looks like it's harmful. Let me adjust this one test to add an actual cert.