Make common_settings flags return platform_common.TemplateVariableInfo

This would allows for `genrule` and friends to consume args as templates, for instance:
```python
load("@bazel_skylib//rules:common_settings.bzl", "string_flag")

string_flag(
name = "myflag",
)

genrule(
name = "myrule",
outs = ["value.txt"],
cmd = """echo $(@myrepo//mypkg:myflag) > $@""",
toolchains = [":myflags"],
)
```

I have a working prototype that just forward flags as templates, and it works well:
```python
load("@bazel_skylib//rules:common_settings.bzl", "BuildSettingInfo")

def _flags_supplier_impl(ctx):
flags = {}
for flag in ctx.attr.flags:
flags[str(flag.label)] = flag[BuildSettingInfo].value
return [
platform_common.TemplateVariableInfo(flags),
]

flags_supplier = rule(
implementation = _flags_supplier_impl,
attrs = {
"flags": attr.label_list(),
},
)
```

Start in rules:common_settings.bzl and inspect the common_settings flags alongside the supplied _flags_supplier_impl prototype. Check how the flags are consumed by genrule and similar rules through platform_common.TemplateVariableInfo. Done means the flags can be used as templates in the shown genrule pattern, with the relevant existing behavior preserved.

Written by the indexing model from the issue text.