java_binary: No way to provide dynamic manifest file content

There seems to be no way to provide dynamic content, like build info into `MANIFEST.MF` file in `java_binary` rule. Only [deploy_manifest_lines](https://www.bazel.io/versions/master/docs/be/java.html#java_binary.deploy_manifest_lines) is supported, where only list of static strings can be provided.

In Buck there is `manifest_file` attribute in [`java_binary`](https://buckbuild.com/rule/java_binary.html) rule where a label can be provided.

What people normally want to achieve is to add build version into manifest file. This can be done (since Bazel 0.3.2) with `--workspace_status_command=./tools/workspace-status.sh` option in `.bazelrc` file and this genrule:

```
genrule(
name = "gerrit_plugin_version",
stamp = 1,
cmd = ("grep STABLE_BUILD_GIT_LABEL < bazel-out/stable-status.txt | " +
"cut -d ' ' -f 2 > $@"),
outs = ['gerrit_plugin_version.txt'],
)
```

However, unless I'm missing something obvious, there is no way to merge this version into `MANIFEST.MF` in `java_binary` rule:

```
java_binary(
name = "gerrit_foo_plugin",
main_class = "Dummy",
# TODO(davido): Move this part into manifest
# when Bazel supports manifest file merging
resources = [":gerrit_plugin_version.txt"],
runtime_deps = ["@guava//jar"],
# Only static key value pairs can be added here.
deploy_manifest_lines = ["bar: baz"],
)
```

`gerrit_plugin_version.txt` is included in the root of the plugin JAR:

```
cat gerrit_plugin_version.txt
5a6f4fd-dirty
```

With the above rules we have this content of `META-INF/MANIFEST.MF`:

```
$ bazel build gerrit_foo_plugin_deploy.jar
INFO: Found 1 target...
Target //:gerrit_foo_plugin_deploy.jar up-to-date:
bazel-bin/gerrit_foo_plugin_deploy.jar
INFO: Elapsed time: 0.551s, Critical Path: 0.46s

$ jar -xf bazel-bin/gerrit_foo_plugin_deploy.jar META-INF/MANIFEST.MF
$ cat META-INF/MANIFEST.MF
Manifest-Version: 1.0
bar: baz
Created-By: blaze-singlejar
Main-Class: Dummy
```

What I would like to be able to have instead, is:

```
$ cat META-INF/MANIFEST.MF
[...]
Implementation-Version: 5a6f4fd-dirty
```

Start with the java_binary rule and its deploy_manifest_lines attribute, then review the workspace-status and genrule example in the issue. Determine how generated content could be merged into META-INF/MANIFEST.MF while preserving existing entries. Done means a java_binary can produce an Implementation-Version value such as 5a6f4fd-dirty in the deployed JAR manifest.

Written by the indexing model from the issue text.