[feature] Refactor imix build configuration

[hulto]

Is your feature request related to a problem? Please describe.
In order to enable more flexible configuration we need to refactor how we configure imix at build time.

Describe the solution you'd like

• Remove the feature flag configuration of transports this should be a run time config and the starting option should be a compile time option.
• Move grpc-doh to a runtime config instead of compile team (remove the grpc-doh feature flag)
• Refactor transport to have required fields callback_uri and interval
• Refactor callback, retry, and proxy to be transport specific instead of agent config specific.
• Remove ActiveTransport layer
• Add multi transport support at build
• Add build file configuration
• Update DNS to use extra field instead of raw DSN
• move config out of pb

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.

[jabbate19]

Final checkbox seems to be a duplicate of #1405 currently being solved by #2383