Add support for LTS releases in the RuntimeCSS Transformer

[schlessera]

Feature description

LTS releases of the runtime can now be requested from the AMP CDN.

The Optimizer transformer that fetches the AMP runtime CSS to inline should be adapted to support this.

See ampproject/amp-toolbox@b154a73

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Acceptance criteria

• Transformer configuration allows for an LTS runtime version to be requested for inlining the CSS.

Implementation brief

• Align to the implementation of the NodeJS version: ampproject/amp-toolbox@b154a73

QA testing instructions

Demo

Changelog entry

[westonruter]

Also mentioned in this PR: #4403

[westonruter]

See also perhaps this master issue or LTS and other release channels: #4600.

[westonruter]

Closing as duplicate of #4600.