load a std.crypto.Certificate.Bundle from file or buffer

[benburkert]

Simplify CA Bundle loading for single files and byte buffers of CA certificate data.

This also fixes a footgun I encountered while using a std.http.Client with a custom ca_bundle:

var ca_bundle: Bundle = .{};
try ca_bundle.addCertsFromFilePath(allocator, cwd, "path/to/cacert.pem");

var http_client: http.Client = .{
        .allocator = gpa,
        .ca_bundle = ca_bundle,
        
          // failing to set this will cause the bundle to rescan from only the system certs,
          // which drops the certs in path/to/cacert.pem above.
        .next_https_rescan_certs = false,
};

Tracking the source now in Bundle won't cause the system certs to load on a rescan:

var bundle: Bundle = try .init(gpa, .{ .file = "path/to/cacert.pem" });
defer bundle.deinit(gpa);

var http_client: http.Client = .{
    .allocator = gpa,
    .ca_bundle = ca_bundle,
};