Add Agemo extension and extension model

[devkelley]

This PR introduces 3 things:

Adds a flexible extension model that allows for services and layers to be added or removed at compile time behind the feature flag.
Relevant files:

• core/common/grpc_interceptor.rs - Modified to allow for a GrpcInterceptor implementation to be passed into the layer rather than the factory.
• core/common/grpc_service.rs
• core/extension/src/extension.rs
• core/extension/src/lib.rs
• core/extension/src/extension_config.rs - tool to load configuration settings from a file. (move to common?)
• core/invehicle-digital-twin/Cargo.toml - Shows how the feature flags are used
• core/invehicle-digital-twin/src/main.rs

Adds a 'managed_subscribe' extension that communicates with the Agemo service to handle subscription requests that need a custom frequency interval. Also introduces an interceptor for this extension that injects the managed_subscribe endpoint for Providers at register time.
Relevant files:

• core/extension/src/managed_subscribe/*
• interfaces/extension/managed_subscribe/v1/managed_subscribe.proto
• external/agemo/*

Adds a sample consumer and provider that utilize the managed_subscribe extension to get some temperature data every 10 seconds.
Relevant files:

• samples/managed_subscribe/*

Additionally, upgrades tonic 0.9.2 > 0.10.0 and prost 0.11 > 0.12

[ashbeitz]

Please take a look at my comments.

[ashbeitz]

One new comment.

[ashbeitz]

Approved.