Implement merging overrides when grouping configs

[DavidSGK]

In #284 we added config grouping which handled merging default values. This PR implements the merging of overrides from the source configs.
Precedence of overrides in the resulting grouped config is based on the input order.
Values of overrides with the same rule strings are merged together.

Given the source configs:

# config-a.star
export(
    Config(
        description = "Config A",
        default = "Hello, world!",
        overrides = [
            ("foo == \"val-1\"", "Goodbye!"),
            ("foo == \"val-2\"", "What?"),
        ],
    ),
)
# config-b.star
export(
    Config(
        description = "Config B",
        default = False,
        overrides = [
            ("foo == \"val-3\"", True),
            ("foo == \"val-1\"", True),
        ],
    ),
)
# config-c.star
export(
    Config(
        description = "Config C",
        default = 123,
        overrides = [
            ("foo == \"val-2\"", 456),
            ("foo == \"val-3\"", 789),
        ],
    ),
)

We get the grouped config:

default_config_v1beta1 = proto.package("default.config.v1beta1")

export(
    Config(
        description = "Grouped from config-a, config-b, config-c",
        default = default_config_v1beta1.Grouped(config_a = "Hello, world!", config_c = 123),
        overrides = [
            ("foo == \"val-1\"", default_config_v1beta1.Grouped(config_a = "Goodbye!", config_b = True)),
            ("foo == \"val-2\"", default_config_v1beta1.Grouped(config_a = "What?", config_c = 456)),
            ("foo == \"val-3\"", default_config_v1beta1.Grouped(config_b = True, config_c = 789)),
        ],
    ),
)

Also tested with string enum configs to confirm.

[lekko-jonathan]

please don't - it's used in one place