[K8SPG-519] Add support for endpointUrl for customExtensions

[spron-in]

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Tell us about the feature

Problem:
When installing custom extensions it is not possible to point to a custom s3 object storage endpoint.

Cause:
The s3 SDK within the extension installer is only minimally configured, currently using default values.

Solution:
Add an optional flag to the extension installer named {{endpoint}} and configure the s3 SDK appropriately.

[https://github.com/percona/percona-postgresql-operator/pull/672|https://github.com/percona/percona-postgresql-operator/pull/672|smart-link]

[https://github.com/percona/percona-postgresql-operator/issues/670|https://github.com/percona/percona-postgresql-operator/issues/670|smart-link]

Link to JIRA