ft: implement pluggable storage

[hauleth]

Change description

Preliminary work for supporting different storages beyond telemetry_metrics_prometheus_core.

What problem does this solve?

Support implementing alternative storage backends (like Peep) for users that require different performance characteristics.

Checklist

• I have added unit tests to cover my changes.
• I have added documentation to cover my changes.
• My changes have passed unit tests and have been tested E2E in an example project.

[hauleth]

@akoutmos I tried running tests locally, but with Elixir 1.16.3 (compiled with Erlang/OTP 25) it fails due to ssl_verify_fun errors, and when I update dependencies it fails for different set of reasons.

[akoutmos]

Thanks for the contribution! I'm away from my machine today but can look deeper into the ssl_verify_fun issues tomorrow. For now I kicked off CI to validate the changes.

[coveralls]

coverage: 78.835% (+0.1%) from 78.698%
when pulling 6a81402 on hauleth:ft/setup-puggable-storages
into 1f6467c on akoutmos:master.