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nfpm: support chaining multiple `InjectNfpmPackageFieldsRequest`s (sorted w/ simple priority integer) (#22864)

This extends then `pants.backend.experimental.nfpm` backend's plugin
API, allowing multiple implementations of this polymorphic-rule plugin
hook (before this change, only one implementation was allowed):

- `inject_nfpm_package_fields(InjectNfpmPackageFieldsRequest) -> InjectedNfpmPackageFields`

Unlike many polymorphic/union rules, each rule runs sequentially instead
of concurrently. This is conceptually similar to middleware, because the
request passed to each rule includes the results of the previous (lower
priority) rule. So, one rule can override or extend fields that were
injected in previous rules, not just the original fields from the
BUILD-file-based `nfpm_*_package` target.

I tried several approaches to sorting the request classes, but most were
error prone or simply didn't work. Heuristic-based sorting proved to be
complex and didn't work in all of my test cases (eg one heuristic was
looking at the class's module to make pants-provided requests lower
priority). A simple `priority` integer is far simpler and more reliable,
so that's what this implements.

Another thing that didn't work was adding `__lt__` as a `@classmethod`
on the abstract `InjectNfpmPackageFieldsRequest`. That fails because
`sorted()` requires an instance method, not a class method. The only way
I found to add an `__lt__` method on a class is via a metaclass. So,
this adds a `_PrioritizedSortableClassMetaclass` to
`InjectNfpmPackageFieldsRequest` allowing for simple `OneRequest <
TwoRequest` sorting of the class type like this:

```diff
     inject_nfpm_config_request_types = union_membership.get(InjectNfpmPackageFieldsRequest)
     applicable_inject_nfpm_config_request_types = tuple(
-        request
-        for request in inject_nfpm_config_request_types
-        if request.is_applicable(target)
+        sorted(
+            request_type
+            for requ... (continued)

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