fix(aes-encryption): support plain txt and url safe base64 strings

[ivankatliarchuk]

Description

Fixes #4975
Fixes #3992

• added tests to cover missing branches for endpoint.{labels,crypto}
• added tests to cover logic when AES keys is base64 form
• added tests to cover current label encryption logic for records
• updated/rephrased documentation
• no changes to business logic

While was forking on a fix, found a bug, where Dynamodb not encrypting labels

external-dns/registry/dynamodb.go

Line 239 in e964a66

statements = im.appendInsert(statements, key, r.Labels)

Trying to keep the scope of this pull request small. And could tweak DynamoDB encryption in follow up if required.

When TXT registry behaves correctly

external-dns/registry/txt.go

Line 270 in e964a66

filteredChanges.Delete = append(filteredChanges.Delete, im.generateTXTRecord(r)...)

Look's like the feature is not in use as was not able to find a single issue to date

Added tests

Checklist

• Unit tests updated
• End user documentation updated

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[k8s-ci-robot]

Hi @ivankatliarchuk. Thanks for your PR.

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