Only select chunks from submissions that match the given key-value set

> As the AI team we are also interested in a new strategy: `SelectOnly`
>
> From https://github.com/channable/opsqueue/blob/master/doc/proposals/strategies_and_metadata.md:
>
> ```
> /// Only select chunks from submissions that match the given key-value set when submission
> SelectOnly(meta_field: String, value: MetaValue, underlying: Strategy)
> ```
>
> For us it would be a key ingredient in implementing blue/green deployments for our AI model. The `meta_field` would then be the model version.

_Originally posted by @SemMulder in [#95](https://github.com/channable/opsqueue/issues/95#issuecomment-4978958634)_

Read doc/proposals/strategies_and_metadata.md for the SelectOnly strategy definition, then locate the existing strategy implementations and metadata handling in the repository. Confirm how an underlying strategy is composed, and define done as selecting chunks only when the submission metadata field matches the requested value.

Scritto dal modello di indicizzazione a partire dal testo della issue.