Add vendored openssl feature

[BlackHoleFox]

OpenSSL is kind of a pain to deal with on macOS, so this tries to make using cargo-instruments more seamless by letting users compile a vendored version of the source instead. This is especially true if you don't use Homebrew, which makes getting the openssl-dev headers much more work.

[cmyr]

Thanks for the PR. I don't fully understand how this works, so maybe you can help me out?

since ssl is only used through cargo, I would expect cargo to provide an appropriate feature to enable vendoring. Is this not the case? An issue I worry about here is that we now need to make sure we keep our version of ssl up to date with what is being used by cargo; maybe not a huge annoyance, but not nothing?

I guess it seems strange to me for us to bring in a crate that we don't use directly, and set features on it, with the intention of changing what is used by a different crate? Can we guarantee that the feature resolver will always handle this correctly?

I also don't think this should be a default feature, unless there's a compelling argument that I'm missing.

[Crosse]

One other reason to bring this in is because it enables those who use MacPorts or pkgsrc (👋) to successfully build this at all. (I just went through this anguish and came up with mostly the same diff.) Without it, there are arcane errors about missing _iconv* symbols coming from libgit2. (It's only after an hour of googling does the connection between the libgit2 errors and openssl become—I won't say apparent, but visible.)

I agree that I don't think it should be a default feature, but having it as an optional feature would be great. I was quite literally coming here to propose that before I found this PR.

Also, thanks for such a wonderful command!

[BlackHoleFox]

Missed the original comment on this one, sorry. I took another look and realized that Cargo actually has its own vendored-openssl feature so I just used that instead of using a direct openssl dependency here. Also made it the non-default since its 2/1 there and I'm good with it not the default.

@cmyr This is ready for review again, and thanks for the nudge to look at cargo itself first.

[cmyr]

Perfect this looks right to me, thanks. I'll cut a release shortly!

[cmyr]

@BlackHoleFox @Crosse published as 0.4.7.