feat(signals): add RxMethod and SignalMethod types to public API

[markostanimirovic]

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PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

RxMethod/SignalMethod` types can be used when specifying the input type of the custom SignalStore feature:

export function withBar() {
  return signalStoreFeature(
    { methods: type<{ foo: RxMethod<number> }>() },
    withMethods(({ foo }) => ({
      bar: () => foo(10),
    })),
  );
}

Does this PR introduce a breaking change?

[ ] Yes
[x] No

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[rainerhahnekamp]

LGTM