This PR adds an External ID Claim Name configuration field to the OIDC
auth source. When set, Gitea uses the specified JWT claim as the user's
`ExternalID` instead of the default `sub` claim.
This PR fixes the bug when migrating from Azure AD V2 to OIDC. When an
admin migrates the same auth source to OIDC, goth's `openidConnect`
provider defaults to using the `sub` claim as `UserID`. However, Azure
AD's `sub` is a pairwise identifier:
> `sub`: The subject is a pairwise identifier and is unique to an
application ID. If a single user signs into two different apps using two
different client IDs, those apps receive two different values for the
subject claim.
https://learn.microsoft.com/en-us/entra/identity-platform/id-token-claims-reference#payload-claims
As a result, every existing user appears as a new account after
migration.
To fix this issue, Gitea should use `oid` claim for `UserID`.
> `oid`: This ID uniquely identifies the user across applications - two
different applications signing in the same user receives the same value
in the oid claim.
Note: The `oid` claim is not included in Azure AD tokens by default. The
`profile` scope must be added to the Scopes field of the auth source.
