Adding this dependency as it going to be used in tests.

This class will be used in two places, so I've decided to make it generic:
- for `ClientRegistrationRepository` bean creation
- for `JwtDecoder` bean creation

The aim of this class is to ensure eventual creation of the object. What that means:
- we will try to create the object on a startup
- in case of failure it will keep retrying the creation in async way until it succeeds

Open question: what is the best value for default retry delay?

Tests two scenarios:
- initialisation on startup
- initialisation after retries

The main purpose of the wrapper is to ensure `ClientRegistrationRepository`
is created rather on a startup or later during retries:
- on a startup it will attempt to create a repository
- if this fails, it'll create `SafeInitProxy` (see previous commits) and pass 2 arguments:
 a supplier to create a `ClientRegistrationRepository` and en error listener (log entry will be present)
- startup will succeed even when IdP is not available
- created proxy will keep attempting to create the repository in the background
- at the same time, if client makes a call (any), `findByRegistrationId` will attempt to get a value
from the proxy (if successful that means the IdP is available). If the value is not present
(meaning IdP is still not available) it'll throw an exception.

Same as for the previous wrapper,the main purpose of this is to ensure `JwtDecoder`
is created rather on a startup or later during retries:

- on a startup it will attempt to create a decoder
- if this fails, it'll create `SafeInitProxy` instance (see previous commits) and pass 2 arguments:
 a supplier to create a `JwtDecoder` and en error listener (log entry will be present)
- this will schedule async retries of creation
- startup will succeed
- created proxy will keep attempting to create the decoder in the background
- at the same time, if client makes a call (any), `decode` will attempt to get a value
from the proxy (if successful that means the IdP is available). If the value is not present
(meaning IdP is still not available) it'll throw an exception that will be returned to the client.

Open question: what the error message should be here?
JwtDecoder relies on ClientRegistrationRepository so I guess the error here should
probably be the same?

…ecoder`

Also, add `.authenticationEntryPoint(authFailureHandler)` (that acts as error handler) to:
`getTokenValidator` and `oidcWebappSecurity`.

This is done so we could customize the error message for the client
and specify the details. This would return our standard error response instead
of a default Spring error response.

As this is a recoverable failure, log level should be "warn" for the repository.
As decoder error is following the repository error, it can be `debug`

This test:
- sets up the application to point to a proxy instead of keycloak first
- verifies that application is up but client request fails with the expected message
- points proxy to keycloak
- verifies that client request succeeds

please note, setting up keycloak takes time.