feat: support multikind casting manifest

[Nageshbansal]

Features

• A casting.yaml can now hold more than one document, one per kind, so an installation and a collection agent live in the same file:

apiVersion: v1alpha1
kind: Installation
metadata:
  name: signoz
spec:
  deployment:
    mode: docker
    flavor: compose
---
apiVersion: v1alpha1
kind: CollectionAgent
metadata:
  name: signoz-agent
spec:
  deployment:
    mode: docker
    flavor: compose

• Documents run in cast order (Infrastructure, Installation, CollectionAgent) no matter how the file orders them, and casting.yaml.lock records them in that order
• Each document keeps its own spec and pours into its own directory
• gauge checks the whole file's tools at once: a tool shared by two documents is checked once, and every missing tool is reported together
• forge writes the lock and the pours only after every document forges, so a failing document leaves the previous generation untouched
• A document without kind still reads as an Installation, so existing casting files keep working
• A duplicate document is rejected, naming the one already declared:

invalid casting file casting.yaml: document 2: Installation is already declared as "signoz": a casting file holds one Installation

There is no casting registered for Infrastructure yet, so a file carrying one is read and ordered but cannot be forged on this branch.

Refactors

• v1alpha1.Kinds() now returns the kinds in cast order
• The config interface and the gauge, forge and cast commands take a list of castings instead of a single one
• Multi-document YAML is read and written through one helper, so errors can point at a document by its position in the file

Tests

• Loader: dispatch by kind, cast order from a file written in reverse, duplicate kinds, error positions, empty and comment-only documents
• Lock: a file of several documents round-trips in cast order
• Reading and writing multi-document YAML streams

Related: https://github.com/SigNoz/platform-pod/issues/2527