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.

[edit] ...and for GIT_OPT_ADD_SSL_X509_CERT, which would make pygit2 dependent on OpenSSL since it requires calling OpenSSL APIs directly.

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.

[ambv]

It turns out GIT_OPT_ADD_SSL_X509_CERT doesn't accept a byte stream of the cert, but instead expects a pointer to an OpenSSL X509 struct. Since I don't want to make pygit2 directly dependent on OpenSSL, I decided to skip support GIT_OPT_ADD_SSL_X509_CERT in this PR. After all, we can use GIT_OPT_SET_SSL_CERT_LOCATIONS instead.

[jdavid]

Thanks!