pdns provider: add optional argument for making the parameter server_id configurable

[romeroalx]

Description

Adds an optional argument to set the server_id to a value instead of always to localhost as it is currently set.

The official documentation of the PDNS API defines that the value of the server_id should almost always be "localhost" except for cases where a proxy is in front of PDNS servers: See https://doc.powerdns.com/authoritative/http-api/server.html#servers.

This PR helps solve those cases where the parameter server_id is different than localhost

Fixes #4752

Checklist

• Unit tests updated
• End user documentation updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: romeroalx / name: Alexis Romero (1a909b8, dad4d13)

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @romeroalx. Thanks for your PR.

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[szuecs]

/ok-to-test

[szuecs]

Running [/home/runner/golangci-lint-1.60.3-linux-amd64/golangci-lint run  --timeout=30m] in [/home/runner/work/external-dns/external-dns] ...
  provider/pdns/pdns.go:46: File is not `gofmt`-ed with `-s` (gofmt)
  	defaultTTL      = 300
  
  Error: issues found
  Ran golangci-lint in 5721ms

[romeroalx]

Running [/home/runner/golangci-lint-1.60.3-linux-amd64/golangci-lint run  --timeout=30m] in [/home/runner/work/external-dns/external-dns] ...
  provider/pdns/pdns.go:46: File is not `gofmt`-ed with `-s` (gofmt)
  	defaultTTL      = 300
  
  Error: issues found
  Ran golangci-lint in 5721ms

Thank you @szuecs , fixed!

[szuecs]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: szuecs

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