Add `{,Dynamic,Unbound}FieldSkipper()`.

It becomes increasingly common to do nothing in a field handler besides tracking
field presence. It is annoying to write lambdas which takes the value, context
parameters, and just returns `absl::OkStatus()`, and it feels uncertain whether
taking and ignoring the value is as efficient as it could (it almost is;
arranging for this to perform varint skipping instead of varint parsing is not
worth complicating the field handler protocol).

In contrast to `DynamicFieldCopier()`, `DynamicFieldSkipper()` field predicate
returns `bool` instead of `std::optional<int>`, because returning a destination
field number is not applicable.

In contrast to `AnyFieldCopier()`, `AnyFieldSkipper()` is not provided because
skipping remaining fields is the implicit default.

In contrast to `UnboundFieldCopier()`, `UnboundFieldSkipper()` can reuse the
implementation of `FieldSkipper()`, because it does not need a destination field
number.

Also, make public `FieldAccepted` and `FieldAcceptedIf` for dynamic field
handlers which carry no extra information about the field being accepted. The
field handler protocol requires an API which fits `std::unique_ptr` and
`std::optional` but not `bool`. These types are extracted from a private
implementation detail of `DynamicFieldHandler()`. They are used by
`DynamicFieldSkipper()` too.

Also, fix embarrassing compile errors in `FieldHandlerWrapper()` for dynamic
field handlers.

PiperOrigin-RevId: 914263217