Allow Maven Embedder to build a model from in-memory buffer

### Description

Maven Embedder exposes API to create a project `Model` from a POM file on a disk. To analyse editor changes, it could be better to have an API that uses `ModelSource` that aloows to read live document contents.

### Use case/motivation

_No response_

### Related issues

_No response_

### Are you willing to submit a pull request?

Yes

Start with the Maven Embedder API that currently creates a project Model from a POM file on disk, then inspect how ModelSource is used. Define the in-memory buffer entry point needed for live editor document contents; done means editor analysis can build a Model without requiring a disk POM.

Written by the indexing model from the issue text.