Make SharedAccessSignatureTokenProvider capable of handing updated creds

[dlstucki]

(New issue based on point 4 in issue #55.)

Consider making SharedAccessSignatureTokenProvider allow updating the credentials. Something similar to this pseudo-code:

public class SharedAccessSignatureTokenProvider extends TokenProvider {

    ... 

    public void set_sharedAccessSignature(String value) { }
    public void set_keyName(String value) { }
    public void set_sharedAccessKey(String value) { }

[dlstucki]

In the meantime here is a work-around:
#55 (comment)
write a class extending TokenProvider that allows changing the key-name+key_value or a signature itself.
Pseudo-code

public class UpdatableSharedAccessSignatureTokenProvider extends TokenProvider {

	// When the credentials change replace this object:  
	private /*SharedAccessSignature*/TokenProvider innerSasTokenProvider;

	. . . 

	public void updateCredentials(String keyName, String keyValue) {
		this.innerSasTokenProvider = TokenProvider.createSharedAccessSignatureTokenProvider(keyName, keyValue);
	}
	public void updateCredentials(String signature) {
		this.innerSasTokenProvider = TokenProvider.createSharedAccessSignatureTokenProvider(signature);
	}

	@Override
	protected CompletableFuture<SecurityToken> onGetTokenAsync(String resource, Duration validFor) {
		return this.innerSasTokenProvider.GetTokenAsync(resource, duration);
	}