Incorrect entry point for tvos_extension

[markvasiv]

The issue

• tvos_extension hardcodes entry point to _TVExtensionMain. I'm not sure where this function comes from but modern tvOS extensions are loaded via Foundation's NSExtensionMain (tested this with top shelf extension targeting tvOS 17).
  https://github.com/bazelbuild/rules_apple/blob/master/apple/internal/tvos_rules.bzl#L1116
• It's possible to workaround it on the client side by redefining TVExtensionMain and jumping over to NSExtensionMain but I think it should be fixed in rules_apple instead (it took me some time to discover why the extension doesn't load).

Proposed change

• I think the safest way to fix this is to add one more parameter to tvos_extension, say legacy_entry_point (default to True to maintain current behavior) and select entry point based on it.
• I can open a PR with the change but I thought I'd ask here first. What do you think about the proposed change?