ci: add remote build

[sinapah]

Combines elements from #405 and adds a workflow input that a caller (from the Otelcol or GA charms) can set to indicate whether the charm release workflow should use local build or remote build. Most observability charm won't need to rely on remote build if they only need to be released for ARM and AMD. For Grafana Agent and Otelcol, we should rely on remote build since they need to also be released for S390x and PPC64EL.

Changes

1. Adds a pack-using-remote-build input to charm-release and _charm-release that is false by default.
2. The caller will set pack-using-remote-build when calling charm-release. The value is propagated to _charm-release
3. The _charm-release WF is adjusted so if pack-using-remote-build is false, it locally packs the charm, similar to what it had done before. Otherwise, it remote builds it.

Testing

PR canonical/o11y-tester-operator#35 in the tester repo tests this workflow. It can test both scenraios (remote vs local build) by setting the pack-using-remote-build parameter when calling the charm-release workflow. When set to false, this workflow uses the AMD and ARM runners to release in parallel. Otherwise, it executes remote-build from the AMD runner.

[MichaelThamm]

I reviewed your o11y-stack PR and it looks identical, therefore LGTM.