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.

All Submissions:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what problem you are trying to solve with this PR?
• Have you added information on what your changes do and why you chose this as your solution?
• Have you written new tests for your changes?
• Does your submission pass tests?
• This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?

[melanger]

relates to SUNET/swamid-satosa#2

[c00kiemon5ter]

is this a convention that is defined somewhere?
(meaning the <attr>#{language} notation)

[melanger]

https://openid.net/specs/openid-connect-registration-1_0.html#LanguagesAndScripts

Human-readable Client Metadata values and Client Metadata values that reference human-readable values MAY be represented in multiple languages and scripts. For example, values such as client_name, tos_uri, policy_uri, logo_uri, and client_uri might have multiple locale-specific values in some Client registrations.

To specify the languages and scripts, BCP47 [RFC5646] language tags are added to Client Metadata member names, delimited by a # character. The same syntax is used for representing languages and scripts for Client Metadata as is used for Claims, as described in Section 5.2 (Claims Languages and Scripts) of OpenID Connect Core 1.0 [OpenID.Core].

If such a human-readable field is sent without a language tag, parties using it MUST NOT make any assumptions about the language, character set, or script of the string value, and the string value MUST be used as-is wherever it is presented in a user interface. To facilitate interoperability, it is RECOMMENDED that any human-readable fields sent without language tags contain values suitable for display on a wide variety of systems.

[melanger]

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