feature: OIDC Provider API

[vinay-gopalan]

Description

The provider API allows Vault admins to configure which Vault issuer, client IDs and scopes are permitted to authenticate via a specific client.

Acceptance Criteria:

A Vault user can perform operations using the provider API as specified in the RFC.

Manual Test

$ vault write identity/oidc/provider/my-provider \
    allowed_client_ids="*" \
    scopes="my-groups" \
    issuer="test-issuer"
Success! Data written to: identity/oidc/provider/my-provider

$ vault list identity/oidc/provider               
Keys
----
my-provider

$ vault read identity/oidc/provider/my-provider   
Key                   Value
---                   -----
allowed_client_ids    [*]
issuer                test-issuer
scopes                [my-groups]

$ vault delete identity/oidc/provider/my-provider 
Success! Data deleted (if it existed) at: identity/oidc/provider/my-provider

$ vault list identity/oidc/provider               
No value found at identity/oidc/provider

$ vault read identity/oidc/provider/my-provider   
No value found at identity/oidc/provider/my-provider

[austingebauer]

LGTM. Thanks!