Allow template_path field on inputs

[jen-huang]

Follow up to elastic/package-spec#79. Kibana needs template_path to ascertain which input template file to read from to build the agent YAML. This PR lets the registry serve that field at the input level, if defined.

[elasticmachine]

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Build stats

• Build Cause: [Pull request #655 updated]

• Start Time: 2020-11-20T19:40:14.858+0000

• Duration: 7 min 49 sec

Test stats 🧪

Test	Results
Failed	0
Passed	165
Skipped	0
Total	165

[jalvz]

thanks!

[mtojek]

Could you please add a new test package with relevant data stream to the testdata/package directory? You can test it with go test .. You will also need to regenerate expected data with go test . -generate.

Please also add a CHANGELOG entry.

[jen-huang]

@mtojek Thanks, I added input_level_templates test package and updated changelog.