Feature Request - Allow file input for `manifest_entries` on `java_export`

[vinnybod]

I have a situation where I want to pass a dynamic value to manifest_entries on java_export such as a version for the application I am building. Right now manifest_entries is a dict, so I can only pass static values (afaik).

It would be nice if manifest_entries could take in a file, so that I can send generated values from a previous build target. I imagine this would work sort of how rules_oci handles remote_tags, etc.

current:

java_export(
    ...
    manifest_entries = {
        "version": "1.0.0"
    }
)

suggested:

stamped_version_file(
    name = "manifest_entries",
    out = "_stamped_version_file.txt",
)

java_export(
    ...
    manifest_entries = "//:manifest_entries"
)