Add support for --@io_bazel_rules_go//go/toolchain:sdk_version flag.

[JamesMBartlett]

What type of PR is this? Feature

What does this PR do? Why is it needed? This PR adds a build flag --@io_bazel_rules_go//go/toolchain:sdk_version, that allows changing which go SDK version to use to build targets. Implementation details:

• Each go_sdk now has config_settings that specify different values of sdk_version. Eg. a go1.17.3 SDK would declare a config_setting for sdk_version="1", sdk_version="1.17", sdk_version="1.17.3", and sdk_version="".
• Then each toolchain that is registered for an SDK will have a config_setting_group constraint that combines the above config_settings. In the example given above, the go1.17.3 SDK would have a toolchain that only gets selected if sdk_version is 1, 1.17, 1.17.3 or the empty string (default).
• If sdk_version flag is not specified, then all SDKs can be selected because of the inclusion of the empty string config_setting in the config_setting_group.

Which issues(s) does this PR fix? Partially #3202 Second diff coming to fully close that issue.

Other notes for review See #3202 for more discussion about this feature.

Working with my employer on signing the CLA at the moment.

Signed-off-by: James Bartlett [email protected]

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[fmeum]

@JamesMBartlett If you rebase on master, you should be able to resolve the CI failure by updating the patch in tests/bcr/bcr.patch - if you have trouble doing that, let me know and I will take care of it.