[Satosa-dev] [microservice] Decide backend by target entity ID

Hi to everybody, I developed a microservice that can map specific SaToSa backends to specific target entity id. A configuration example can be this: ```` module: satosa.micro_services.custom_routing.DecideBackendByTarget name: TargetRouter config: target_mapping: "http://idpspid.testunical.it:8088": "spidSaml2" "http://strangeIDP.testunical.it:8081/saml2/metadata": "strangeSaml2" ```` I needed a backend routing based on the target entity ID because I have some SAML2 IDP that only accepts highly customized authn request and metadata. An example would be SPID italian federation, through which my organization will federate soon with SaToSa. Another example could be the need to use different configurations, like enc and digest algorithms, depending by target IDP. I was looking into DecideBackendByRequester microservice but soon I realized that it was made for different goals, in it the subjects are the requester entity ID and not the target entity ID. As you can see in https://github.com/IdentityPython/SATOSA/pull/220 I made a single branch to pull only this feature. I'm also curious about SaToSa milestone, which are the features in development status, which will compose the next release and another question about the possibility to have a dev branch to do PR on it. I don't know if this microservice could sound useless to you, I searched a lot before programming it and I hope to have done a middleware that could be usefull for the SaToSa community. Hope to hear your comments soon