PMM: build pmm-server for arm64

[EvgeniyPatlan]

This adds a native arm64 build of pmm-server v3, following the same layout we already use for the client:

• pmm3-server-autobuild becomes a parent job that runs two new jobs (pmm3-server-autobuild-amd and pmm3-server-autobuild-arm) in parallel and merges their images into a multi-arch manifest with docker buildx imagetools. It still exports TIMESTAMP_TAG, so pmm3-release-candidate doesn't need any changes.
• watchtower is built for both arches too - the binary cross-compiles, the image is assembled per arch and merged the same way. It runs next to the server on every staging instance, so an arm server host can't live without it.
• pmm3-api-tests got an AGENT_ARCH parameter and the server parent now triggers it for both arches with the image it just built.
• pmm3-image-scanning scans both platforms of the manifest and archives per-arch reports.
• pmm3-aws-staging-start got a SERVER_ARCH parameter (arm64 = t4g.xlarge). runSpotInstance takes the arch as a second argument, default is x86_64 so nothing changes for existing callers. The arm64 worker AMI already exists (same pmm-worker-3 tag, the packer template builds both since a while).
• package testing jobs and their matrices pass SERVER_ARCH through, so the arm matrix now runs client and server both on arm.
• setupPMM3Client: latest-tarball resolves to the arm client tarball on aarch64 hosts, before it always downloaded the amd64 one.

All of this was tested from a test branch with separate -test job copies and -test docker tags before opening this PR: the multi-arch manifest builds, api tests are green on both arches, an arm64 staging instance comes up with server+watchtower+client working, package tests ran against it.

Depends on percona/pmm#5630 - without the arch-namespaced S3 build cache the arm build downloads cached x86_64 rpms and skips building. Merge that one first.

Two new jobs need to be registered when this merges: pmm3-server-autobuild-amd and pmm3-server-autobuild-arm (pipeline from SCM, master, script paths match the file names).

Not in this PR, follow-ups:

• pmm3-client-repo-push uploads both arch tarballs to TESTING/pmm/ under the same file name, so amd64 and arm64 overwrite each other on every RC. Needs an arch-aware destination.
• rc-testing lanes and nightly UI runs against an arm server.
• arm64 AMI wiring in pmm3-ami (the packer side is already in Prepare server build for arm64 percona/pmm#5630).

[ademidoff]

Let's remove everything related to Watchtower as it is being discontinued.

[EvgeniyPatlan]

@ademidoff it is just test job so it really now copies the old behavior. Sure we can remove this job. But I suggest keeping it for now so it would be possible to test with and without watchtower,