feat: add metadata_store to context

[kofzera]

Added a function that collects metadata for swamid-satosa metainfo plugin for non-SAML backends. The metadata is collected into dictionary and added to the context. The functionality added is similar to saml backend where the context is decorated with metadata of mdstore type.

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[melanger]

relates to https://github.com/SUNET/swamid-satosa/pull/2

[melanger]

@c00kiemon5ter Hi, can you please merge this? It should not break anything as it only adds metadata to the Context.