fix(rds): add writer instance id to database cluster's instance ids

[argarinpauljohn]

Adding the writer instance's id to the instance ids being returned. Currently it only returns just the readers'.
Closes #25947.

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[gitpod-io]

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[corymhall]

Good catch! Can you also add a unit test?

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[amscheaws]

Probably also need to update the documentation for the cluster's property? It says,

Identifiers of the replicas.

Should this be,

Identifiers of the writer and readers.

Thanks.

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