…v3.49.1 (#3011)

Co-authored-by: Phong Chuong <[email protected]>

…v3.49.1 (#3011)

Co-authored-by: Phong Chuong <[email protected]>

…on image (#3011)

This PR moves `java.py` and the minimum required dependencies from
[synthtool](https://github.com/googleapis/synthtool/tree/master/synthtool).

It contains the changes of @JoeWang1127's
googleapis/synthtool@696c4bf

### Details of code transfer

Some files have been simplified and others simply verbatim-copied. Here
is a detailed list of changes (the paths are relative to
`library_generation/owlbot`:
* __`synthtool/__init__.py`__: No changes made. This is the module
initialization configuration to allow statements such as `import
synthtool as s` combined with `s.copy`, `s.replace`, etc. Without this,
we would have to use `import synthtool.transforms as s`.
* __`synthtool/_tracked_paths.py`__: No changes made. It allows to use
relative paths internally when working with library files.
* __`synthtool/gcp/common.py`__: Simplified to use java-only functions.
This file originally had a set of functions to support postprocessing of
languages written in multiple languages. The most important function is
`common_templates`, which renders the templates for the library (e.g.
workflow files, kokoro files). Note that `common_templates` was modified
in order to require and only allow specifying the path to the templates
via the `SYNTHTOOL_TEMPLATES` env var, as opposed to its original
support of 3 separate ways, including cloning synthtool and reading the
templates from there, because this is now an internal detail of how the
`library_generation` image will work.
* __`synthtool/gcp/samples.py`__: No changes made. This is a helper to
obtain path and metadata about the generated samples of a library. It is
then used [when rendering the
README](https://github.com/googleapis/sdk-platform-java/blob/4c5f932191dfb792b370c8446e8a018a9ef56ffb/library_generation/owlbot/templates/java_library/README.md?plain=1#L141-L150).
* __`synthtool/gcp/snippets.py`__: No changes made. Similar to
`samples.py` in the way it's
[used](https://github.com/googleapis/sdk-platform-java/blob/4c5f932191dfb792b370c8446e8a018a9ef56ffb/library_generation/owlbot/templates/java_library/README.md?plain=1#L68-L80)
to render a library's README
* __`synthool/languages/java.py`__: Small modifications around the fact
that we dropped several files (e.g. use `gcp.common.CommonTemplates`
instead of `gcp.CommonTemplates` to save us from an extra
`gcp/__init__.py`).
* __`synthtool/sources/templates.py`__: No changes made. Internally used
by `common_templates`. Contains the underlying usage of jinja2 to render
the templates.
* __`synthtool/transforms.py`__: No changes made. Contains a few
functions that are commonly used by `owlbot.py` files
([example](https://github.com/googleapis/java-bigtable/blob/45732201880a13eeced3d0332bd172aae0f73dbe/owlbot.py#L50))

### Changes in Dockerfile
We will not clone `synthtool` anymore. We will instead install it as a
separate package whose source code is within sdk-platform-java.