start API improvements

[seemk]

• instrument script now uses the start call instead of duplicating the logic.
• New option for start:resource - a method which is called with the detected environment resource. Users can return their own resource from this function, which gets passed to all signals (instead of doing it separately for all signals - where the API for it differed).
• New option for start: realm - when specified the realm is passed to the underlying signals. Only applicable for traces and metrics at the moment as logs and profiling data can't be sent directly over OTLP.
• Resource detection is now cached. Previously each signal called the same detection routines.
• When http/protobuf (the default setting) is enabled and endpoint option is set, signals now attach the resource path to the endpoint. E.g. start({ endpoint: 'http://localhost:4318' }); means /v1/traces, /v1/metrics and /v1/logs resource paths are automatically appended.