Templates match indices with date math expressions

[danhermann]

The root cause of the bug identified in #69727 (reported independently by several users) is that the findV?Templates methods in MetadataIndexTemplateService do not resolve index names with date math expressions before attempting to match them. The reproduction scenario in #69727 demonstrates one flavor of this problem. Before 7.9, an ingest pipeline with a date_index_name processor would (accidentally) work around that problem, but the simple statement below would fail to execute the final pipeline in any ES version (assuming a template that defines a final pipeline for an index pattern of test-*):

# url-encoded form of <test-{now/d}>
POST /%3Ctest-%7Bnow%2Fd%7D%3E/_doc 
{
  "field": "value"
}

A change to the way ingest pipelines work in 7.9 caused the date_index_name scenario to fail as well. This change resolves both problems.

Fixes #69727.

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)

[danhermann]

@elasticmachine update branch

[dakrone]

LGTM

[danhermann]

Thanks, @dakrone!

[gwbrown]

@danhermann Looks like we're good to remove backport pending from this PR, yes?