Agent: Produce a simple, minimal code agent with opinionated defaults

[Steve Gordon]

We now have a basic POC completed for an Agent distribution. This identified some limitations in the design of the API surface which should be addressed. For example:

• We should not force our notion of a Service \ Resource on the consumer. Instead, we should provide hooks such that the built-in ResourceBuilder can be accessed by consumers for more advanced scenarios.
• We should register default sources to listen to a recommended set of ActivitySource providers.
• We should export by default using OTLP to an Elastic backend.
• We should support consumers registering processors and exporters which we then apply to the builders in the appropriate order.

[Steve Gordon]

We have an initial agent concept that configures common defaults and exports via OTLP.