Allow authentication for specific implementations of Public Key Infrastructure

Overview

Aim of this change is allowing specific implementations for authentication based on security tokens.
Nowadays, user_saml app accepts two types of login: saml and environment-variable. But there are different sets of OIDs for different public key infrastructures. Then, there are different strategies of authentication.
The proposal of implementation is allowing that programmers use properly strategy for public infrastructure public they are working with.
It's no intention to discard current implementation, but to allow that it can be adapted according reality of environment.

Steps to reproduce

1. Extends OCA\User_SAML\Strategies\AuthenticationAbstractPKIStrategy.
2. Extends OCA\User_SAML\UserBackend.

Expected behaviour

A user logs in Nextcloud using a security token.

Actual behaviour

Nextcloud does not support login for any security token.

Server configuration

It is necessary to config certificate in web server. Below are items that must to be set in apache site configuration file.
`
<VirtualHost *:443>

UseCanonicalName On
ServerName [SERVERNAME]

SSLEngine on
SSLProxyEngine on

SSLCertificateFile      [CRT FILE]
SSLCertificateKeyFile   [KEY FILE]
SSLCACertificateFile    [CRT FILE]
SSLVerifyDepth 3
SSLOptions +ExportCertData +StdEnvVars

# Initialize the special headers to a blank value to avoid http header forgeries
RequestHeader set SSL_CLIENT_CERT ""
RequestHeader set SSL_CLIENT_VERIFY ""

SSLVerifyClient none
<Location "/index.php/apps/user_saml/saml/login">
    RewriteEngine on
    RewriteCond %{HTTPS} off
    RewriteRule (.*) https://%{HTTP_HOST}%{REQUEST_URI} [L]

    SSLVerifyClient optional
    SSLVerifyDepth 3
    SSLOptions +ExportCertData +StdEnvVars

    RewriteCond %{SSL:SSL_CLIENT_VERIFY} !^SUCCESS$
    RewriteRule .* /apps/user_saml/ssl_error.html [R=302,L]
    RequestHeader set SSL_CLIENT_CERT "%{SSL_CLIENT_CERT}s"
    RequestHeader set SSL_CLIENT_VERIFY "%{SSL_CLIENT_VERIFY}s"
</Location>

SSLProtocol All -SSLv2 -SSLv3

SSLHonorCipherOrder  on

SSLCipherSuite [CIPHER]

SetEnv nokeepalive ssl-unclean-shutdown

`