feat: add configurable certificate mount path support

[Yuri0405]

Summary

Adds support for configurable certificate mount paths via the autocert.step.sm/mount-path annotation, enabling integration with Istio and other applications that require certificates in specific locations.

Name of feature:

Configurable Certificate Mount Path

Pain or issue this feature alleviates:

• Enables integration with Istio service mesh, which expects certificates at specific paths
• Provides flexibility for applications that need certificates mounted at custom locations
• Eliminates need for symlinks or file copying to work with different certificate expectations

Why is this important to the project:

• Expands autocert compatibility with service mesh technologies like Istio
• Makes autocert more flexible and adaptable to different deployment scenarios
• Addresses a specific user request for Istio integration (Istio certs ? #256)

Is there documentation on how to use this feature? If so, where?

Yes, comprehensive documentation is provided in CUSTOM_MOUNT_PATH.md including:

• Usage examples with annotations
• Custom controller image build instructions
• Deployment update procedures
• Verification steps

In what environments or workflows is this feature supported?

• All Kubernetes environments (local and remote clusters)
• Particularly useful for Istio service mesh deployments
• Compatible with existing autocert functionality
• Works with minikube, kind, GKE, EKS, AKS, etc.

In what environments or workflows is this feature explicitly NOT supported (if any)?

• Requires custom controller image build until officially released
• No known compatibility restrictions otherwise

Supporting links/other PRs/issues:

Fixes #256

Usage Example

apiVersion: apps/v1
kind: Deployment
metadata:
  name: my-app
spec:
  template:
    metadata:
      annotations:
        autocert.step.sm/name: my-app.default.svc.cluster.local
        autocert.step.sm/mount-path: /etc/ssl/certs
    spec:
      containers:
      - name: app
        image: my-app:latest