feat: Add a way to select a specific branch before building the devcontainer

### Description

Improve feature parity with `github codespaces`.

The goal is to add a new `coder_parameter` called `target branch` to workspace templates.
It will let the user select a branch from which to build a devcontainer.

Unlike the devcontainer cli, the `git clone` happen in the `enbuilder` container.
So I think it is the responsibility of `envbuilder` to checkout to the target branch **before building the container**.

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PS: The [workaround in readme.md](https://github.com/coder/envbuilder?tab=readme-ov-file#git-branch-selection) does not work on all platforms.

Read the readme.md Git branch selection workaround first, then trace the coder_parameter handling and the envbuilder checkout flow. Done means a target branch can be selected in workspace templates and is checked out before the devcontainer is built, including platforms where the README workaround fails.

索引モデルが issue の本文から書いたものです。