Introduce Config Provider as an option to set the initial configuration and update it.

[RonFed]

This PR adds a config.Provider interface which supplies an initial configuration and allows modification of the instrumentation configuration in runtime.
Currently, the configuration includes:

• enable/disable of each instrumentation library.
• global default disable - which allows setting the default behavior for enabling/disabling instrumentations.

Future additions to the configuration might be:

• Sampling
• Collecting extra attributes which are not collected by default.

Future implementations of the Provider interface might include:

• Providing initial configuration based on environment variables.
• Providing initial configuration and updates based on a configuration file.