Add ttl, update docs and move to declarative API

[renancaraujo]

Move to an API that allows composability:

Simple:

// this is unchanged, not a shorthand anymore, the default:
CachedValue(() => factorial(originalValue));

// from:
CachedValue.simple(() => factorial(originalValue));

// to:
CachedValue(() => factorial(originalValue));

Dependent:

// from:
final factorialCache = CachedValue.dependent(
  on: () => originalValue,
  compute: () => factorial(originalValue),
);

// to:
final factorialCache = CachedValue(
  () => factorial(originalValue),
).withDependency(() => originalValue);