Improve custom code hooks in Program.cs

[danielskovli]

Description

As it stands, the RegisterCustomAppServices methodology in Program.cs doesn't quite capture all use cases. An example of this is when an app requires integration with key vault.

Slack thread: https://digdir.slack.com/archives/C076GU2QHCG/p1737726498755789

Propose a new structure of this file which simplifies boxing of custom code, while allowing enough flexibility to satisfy common use-cases.

Acceptance Criteria

• The new structure adheres to the same order of operations as the current template (eg. custom code runs before AddAltinnAppServices, etc)
• All supported customisations should be possible from within a clearly labelled area of Program.cs, ideally a single callback or a well defined hooks interface

[danielskovli]

Regarding a future desire to automate Key Vault injection, consider modifying values.yml as required for secrets.