Validate that custom ilm_policies are defined in the package

[jsoriano]

Data streams can have custom ILM policies since #90, but nothing is checking that these policies actually exist.

This new validation checks that the ILM policy is defined in the same data stream where it is used, what helps to have self-contained packages, and is coherent with the current use of this feature in existing packages.

Fixes #92.

[elasticmachine]

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• Start Time: 2022-09-14T09:59:38.255+0000

• Duration: 7 min 11 sec

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Test	Results
Failed	0
Passed	626
Skipped	0
Total	626

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[elasticmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (7/7)	💚
Files	68.0% (17/25)	👎 -2.833
Classes	76.471% (26/34)	👎 -2.317
Methods	56.075% (60/107)	👎 -2.749
Lines	42.184% (537/1273)	👎 -2.409
Conditionals	100.0% (0/0)	💚

[jsoriano]

Migration docs added in elastic/elastic-package#977.

[mrodm]

👍 Added some non-blocking comments