golang: add versioned package for 1.25

golang: add versioned package for 1.25

Add versioned package for 1.25 to enable having multiple host go
versions side by side.

Set default version to 1.25 in golang-values.mk

Add unversioned dummy package to allow go-based packages to continue
using the default go host version. Packages can use it by specifying:

PKG_BUILD_DEPENDS:=golang/host

or use a specific version out of the ones that are available in that
branch by specifying:

PKG_BUILD_DEPENDS:=golang1.25/host

Host go is exposed to each package through PATH set in
GO_PKG_BUILD_CONFIG_VARS and GO_PKG_VARS.

Target go is installed through alternatives with the default version
having higher priority.

Newer versions can reuse older ones as bootstraps by setting
GO_BOOTSTRAP_VERSION package variable to older version, e.g.:

GO_BOOTSTRAP_VERSION:=1.24

All subpackages provide suffix-less names, e.g. golang, golang-src, etc.
Default versions are marked as default variants.

Link: https://github.com/openwrt/packages/pull/28309
Signed-off-by: George Sapkin <redacted>