Extra alpm feature

Hello! I tried to figure out if it was possible to just directly submit an MR for this, but that requires being a member of a team, so, I'll open an issue instead.

It would be nice to offer a generate feature that forwards to alpm/generate, which generates the alpm bindings from the currently installed version of pacman instead of the pre-generated ones. This is useful for Arch forks like ALARM which may have a slightly out-of-date or patched pacman version, so that you can still build arch-audit for those systems.

Obviously, a separate pacman repository will have slightly different builds where there could be more or fewer issues, but it's still useful to compare the audit information for mainline Arch with these forks, since most issues that apply to the main packages will likely apply to the forked versions, even if the issue list isn't 100% representative. (Which is still the case for mainline Arch anyway, since no security database is perfect.)

Specifically, I suggest this change:

diff --git a/Cargo.toml b/Cargo.toml
index d6afdef..ce487bf 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -38,6 +38,7 @@ anstyle = "1.0.8"
 tempfile = "^3.12"
 
 [features]
+generate = ["alpm/generate"]
 git = ["alpm/git"]
 
 [profile.release]