feat(Google CloudDNS): add routing policy support

[Dadeos-Menlo]

Description

Google's CloudDNS service supports routing policies (i.e. geolocation and weighted-round-robin based query responses) but the current implementation of the "google" provider does not support managing such routing policy enabled resource record sets.

The proposed changes include:

• Introduction of a plan.Changes.All() function, and associated plan.RRSetChange structure, for representing changes to be applied to a managed domain as an ordered sequence of changes grouped by resource record set
• Refactoring of the "google" provider unit-tests in order to express test expectations in terms of the state of managed domains' resource record sets
• Modification of the "google" provider to introduce support for handling routing policy enabled resource record sets

Notes:

• Routing policies introduce additional dimensions to resource records sets (e.g. the regular dimensions of DNS name and record type are augmented by location or weighted set entry) the Endpoint representation of resource records uses the, somewhat AWS Route53 centric, SetIdentifier field to support any additional dimension[s]
• AWS' Route53 service maintains distinct objects for each routing policy enabled resource record whereas Google's CloudDNS service maintains all of the routing policy configuration within a single ResourceRecordSet object, this data-structure necessitates a read-modify-write approach when attempting to alter a routing policy enabled sub-record
• The proposed changes introduce a number of "google" provider specific annotations, the getProviderSpecificAnnotations(…) function is modified accordingly but the changes proposed in pull-request 4875 are considered to be a preferable approach

Checklist

• Unit tests updated
• End user documentation updated

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @Dadeos-Menlo. Thanks for your PR.

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[mloiseleur]

/ok-to-test

[mloiseleur]

/retitle feat(Google CloudDNS): add routing policy support