Backport of #51233 to the seven dot x branch.
Tries to load a `Mapper` instance for the mapping snippet of a dynamic template.
This should catch things like using an analyzer that is undefined or mapping attributes that are unused.
This is best effort:
* If `{{name}}` placeholder is used in the mapping snippet then validation is skipped.
* If `match_mapping_type` is not specified then validation is performed for all mapping types.
If parsing succeeds with a single mapping type then this the dynamic mapping is considered valid.
If is detected that a dynamic template mapping snippet is invalid at mapping update time then the mapping update is failed for indices created on 8.0.0-alpha1 and later. For indices created on prior version a deprecation warning is omitted instead. In 7.x clusters the mapping update will never fail in case of an invalid dynamic template mapping snippet and a deprecation warning will always be omitted.
Closes#17411Closes#24419
Co-authored-by: Adrien Grand <jpountz@gmail.com>
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