Allow to enable/disable scripting based on their source (where they get loaded from), the  operation that executes them and their language.

The settings cover the following combinations:

- mode: enabled, disabled, sandboxed
- source: indexed, dynamic, file
- engine: groovy, expressions, mustache, etc
- operation: update, search, aggs, mapping

The following settings are supported for every engine:

script.engine.groovy.indexed.update:    sandbox/enable/disable
script.engine.groovy.indexed.search:    sandbox/enable/disable
script.engine.groovy.indexed.aggs:      sandbox/enable/disable
script.engine.groovy.indexed.mapping:   sandbox/enable/disable
script.engine.groovy.dynamic.update:    sandbox/enable/disable
script.engine.groovy.dynamic.search:    sandbox/enable/disable
script.engine.groovy.dynamic.aggs:      sandbox/enable/disable
script.engine.groovy.dynamic.mapping:   sandbox/enable/disable
script.engine.groovy.file.update:       sandbox/enable/disable
script.engine.groovy.file.search:       sandbox/enable/disable
script.engine.groovy.file.aggs:         sandbox/enable/disable
script.engine.groovy.file.mapping:      sandbox/enable/disable

For ease of use, the following more generic settings are supported too:

script.indexed: sandbox/enable/disable
script.dynamic: sandbox/enable/disable
script.file:    sandbox/enable/disable

script.update:  sandbox/enable/disable
script.search:  sandbox/enable/disable
script.aggs:    sandbox/enable/disable
script.mapping: sandbox/enable/disable

These will be used to calculate the more specific settings, using the stricter setting of each combination. Operation based settings have precedence over conflicting source based ones.

Note that the `mustache` engine is affected by generic settings applied to any language, while native scripts aren't as they are static by definition.

Closes elastic#6418
Closes elastic#10116

…s argument

… with the usual boolean values, plus `sandbox`

…ativeNames on ScriptedOp

…nguage

…nguage