{r,m}ctx.download*: report size of downloaded file (https://github.com/bazelbuild/bazel/pull/29463)
### Description
Adds a new structure field `size_bytes` in the return value of `rctx.download` (and friends) containing the file size of the downloaded file.
### Motivation
This is a useful piece of metadata Bazel provides to repository rules and module extensions. Motivation for adding this: I need the ability to annotate BUILD files with sizes of downloaded files in rules_runfiles_group (https://github.com/hermeticbuild/rules_runfiles_group). This makes it possible to merge runfiles groups based on their "weight", which may include actual file sizes of *_import targets (java_import, cc_import, ...).
### Build API Changes
It does affect the Build API, but is backwards compatible (adding a new field).
Existing users will not be impacted.
### Checklist
- [x] I have added tests for the new use cases (if any).
- [x] I have updated the documentation (if applicable).
### Release Notes
RELNOTES[NEW]: {r,m}ctx.download* report the size of downloaded files
RELNOTES: Addition to the user-facing Build API
Closes #29463.
PiperOrigin-RevId: 910706646
Change-Id: I980cdee803810e1fb6222f69782fcad2648af68d
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