feat(signals): add resource extensions

[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:

Closes #5126

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[rainerhahnekamp]

I would directly address here the open question, namely the difference to the native snapshots. To me the additional benefits are the ability of having

1. A WritableSignal
2. The option to define them globally.

So I would rephrase it like

Suggested change

	The `extendResource` function, together with the built-in extensions, addresses both cases without losing access to the APIs of more specific resource types, such as `WritableResource`.
	Unlike the native `resourceFromSnapshots` function, which downcasts the output to a base `Resource`, `extendResource` can be configured globally and fully preserves the capabilities of writable types like `WritableResource`.

[markostanimirovic]

I'd avoid mentioning resourceFromSnapshot in case this limitation gets solved on the Angular side in the meantime.