[7.17] Allow ILM to transition to implicit cached steps (#91779)

[andreidan]

ILM tries to honour the cached phase however, some steps are implicit
(e.g. injected actions or the terminal policy/phase step)
Currently, ILM would throw an exception if the currently cached phase was removed:

step [{"phase":"warm","action":"complete","name":"complete"}] for index [index]
with policy [my-policy] does not exist

ILM would currently also throw an exception if the next step was an
implicit one and the phase was removed from the underlying policy e.g. if the
index is on the migrate step and looking to transition to the
check-migration step, whilt the warm phase doesn't exist in the
policy anymore

step [{"phase":"warm","action":"migrate","name":"check-migration"}] for index
[index] with policy [my-policy] does not exist

This fixes these scenarios by enhancing the
PolicyStepsRegistry#parseStepKeysFromPhase method to compute all the
steps in the phase (including all implicit steps)

Backport of #91779