[META] Integrate with OpenSearch Settings

[dbwiddis]

Is your feature request related to a problem?

Extensions may have their own settings that must be registered with OpenSearch.

Extensions may make use of OpenSearch settings and need to subscribe to changes in those settings.

What solution would you like?

• A local copy of OpenSearch settings populated at initialization
• A Setting class porting the relevant settings creation portions of Setting
• Relevant transport-related portions of the individual Setting subclasses
• Relevant transport-related portions of the overall WriteableSetting
• A getSettings() extension point on the Extension interface
• A handler for registering settings obtained from the above interface
• A handler for subscribing to and receiving setting updates

What alternatives have you considered?

None; the above most closely resembles the Java SDK implementation which required multiple iterations to get right.

Do you have any additional context?

The Java WriteableSetting constructor takes a Setting<T> object as a parameter with this Setting used in the extension. Given that we are not importing the Python class it is entirely reasonable to combine the functionality of the two classes.

[dblock]

@dbwiddis update this one with current status?

[dbwiddis]

It's up to date. The third bullet is partially complete but still doesn't have transport for the superclass. I'll split it into two parts to make it more clear.