[bazel] Add component "extensions" rule. (#6086)
[bazel] Add component "extensions" rule. (#6086)

Since our extensions should generally follow the file system convention
for naming, we can create a custom bazel rule that reduces some
boilerplate code.

----

Example
-------

**Before**

```
mdc_objc_library(
    name = "Theming",
    srcs = native.glob(["src/Theming/*.m"]),
    hdrs = native.glob(["src/Theming/*.h"]),
    includes = ["src/Theming"],
    visibility = ["//visibility:public"],
    deps = [
        ":ActionSheet",
        ":ColorThemer",
        ":TypographyThemer",
        "//components/schemes/Container",
    ],
)
```

**After**

```
mdc_extension_objc_library(
    name = "Theming",
    deps = [
        ":ActionSheet",
        ":ColorThemer",
        ":TypographyThemer",
        "//components/schemes/Container",
    ],
)
```
material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios,material-components/material-components-ios