refactor secdist: get rid of DefaultSecdistProvider component

[mnink275]

This PR solves the issue #547
The main changes are made in the directory: core/{include/userver,src}/storages/secdist/

• The components::DefaultSecdistProvider component has been removed.
• Added components::SecdistComponentBase - base component for all secret distributors. The default implementation of this class is components::Secdist - ready-to-use component.
• Implementations of the storages::secdist::SecdistProviderBase class are now used to store the secdist configuration.

Thus it is still possible to implement your own secret distributor by implementing the base classes components::SecdistComponentBase and storages::secdist::SecdistProviderBase.

[apolukhin]

Wow! You've done much more than we were expecting!

Unfortunately, in our internal codebase we already have a lot of custom providers. It is almost impossible for us, to make a migration in one go - hundreds of services should be adjusted and changed simultaneously.

Could you please split this PR into multiple?

First PR should preserve the provider config parameter and the SecdistProvider hierarchy. The DefaultSecdistProvider component should remain in code but not used in userver static configs. provider option should become empty by default, and in that case the whole functionality is implemented by the Secdist itself. That way we could merge the PR without affecting the services, remove the default-secdist-provider usage in hundreds of services.

After that, the DefaultSecdistProvider could be removed by a separate PR. And after that, we can take a second look on new hierarchy of providers (however, I think that it is fine to leave the provider to customize the Secdist in complicated cases).