[extension/zpages] Allow to register additional zpage endpoints programmatically on the zpages extension

[atoulme]

Is your feature request related to a problem? Please describe.
We have customized our zpages usage by adding additional endpoints. We would like to make this a programmatic approach that is recognized as valid with the collector.

Describe the solution you'd like
Allow an API that registers additional pages to the zpages extension programmatically.

Describe alternatives you've considered
Drop our extensions to zpages.

Additional context
https://github.com/signalfx/splunk-otel-collector/blob/main/internal/configconverter/config_server.go#L79

[yurishkuro]

Do zpages always require an actual page, or these additional endpoints be just admin/debug-style API handlers?