Add configuration modifiers to a specific target

Modifiers can be added to individual targets via the modifiers attribute which is exposed by all rules (this is handled by Buck2 itself, so rule authors do not have to add it themselves).

For example, lets assume that we have a release_package rule that references various deps and packages them, and that our PACKAGE file defaults our build mode to debug. We could add the release modifier to our artifact target to build all dependencies in release mode rather than debug:

release_package(
    name = "my_package",
    binaries = [
        ":my_cxx_bin",
        ":my_rust_bin",
    ],
    modifiers = [
        "//constraints:debug",
    ],
)

As another example, imagine that we have a constraint that controls whether we use real or simulated IO:

constraint_setting(name = "io_mode")

constraint_value(
    name = "real_io",
    constraint_setting = ":io_mode",
)

constraint_value(
    name = "simulated_io",
    constraint_setting = ":io_mode",
)

The default IO mode would be real_io, but we would like to override it for a specific test:

go_test(
    name = "simulated_io_test",
    srcs = ["test.go"],
    modifiers = ["//constraints:simulated_io"],
)