🌱 add merge strategy markers

[apricote]

What this PR does / why we need it:

This commit adds merge strategy markers to our v1alpha6 API. This is required for Server-Side Apply (SSA) to work correctly. SSA is used in Cluster API v1.2, and not having these markers might cause issues when using ClusterClass patches.

This only adds a subset of markers, where it was possible. I skipped adding markers for two cases:

• We have many slices that do not have any proper map keys, as all fields are optional. This includes mostly "filter" structs, for example NetworkParam and SecurityGroupParam.
• The selected map key may not be optional (omitempty). If it is, it can not be used as the map key. This applied to structs AddressPair and ExternalRouterIPParam.

Further documentation:

• CAPI v1.2 provider migration: https://cluster-api.sigs.k8s.io/developer/providers/v1.1-to-v1.2.html#required-api-changes-for-providers
• SSA Markers: https://kubernetes.io/docs/reference/using-api/server-side-apply/#merge-strategy

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged): Related to #1282

Special notes for your reviewer:

/hold

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[tobiasgiese]

/approve /lgtm

Thanks for implementing the merge strategy markers 👍🏻 one step further towards CAPI v1.2

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: apricote, tobiasgiese

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• ~~OWNERS~~ [tobiasgiese]

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[jichenjc]

/lgtm /hold cancel