Term Vectors/MLT Query: support for different analyzers than default at field

This adds a `per_field_analyzer` parameter to the Term Vectors API, which
allows to override the default analyzer at the field. If the field already
stores term vectors, then they will be re-generated. Since the MLT Query uses
the Term Vectors API under its hood, this commits also adds the same ability
to the MLT Query, thereby allowing users to fine grain how each field item
should be processed and analyzed.

Closes #7801