Consolidate mount configuration fields in Session Create API

## Motivation  

Currently, the Session Create API has two separate fields for handling volume mounts:

- `mounts`: Specifies the vfolder names/IDs to be mounted
- `mount_map`: Specifies the mount target paths for the vfolders

This separation creates unnecessary complexity in the API schema. These fields serve closely related purposes and should be consolidated into a single field to improve API usability and maintain cleaner schema design.

## Objective  

Combine both fields into a single field that can handle both the vfolder identifier and its mount path in one structure.

## Expected Sub Issue

- Update API schema to add a new field that accepts a list of mount configurations in JSON format
- Update `ai.backend.manager.models.vfolder.prepare_vfolder_mounts()` to handle the new field structure
- Mark `mount_map` and `mounts` fields as deprecated
- Update SDK and CLI to support the new mount configuration field
- Update API documentation with the new field format and deprecation notices
- Create migration guide for existing API consumers to adopt the new field

JIRA Issue: BA-656