Introduce `SourceContext` to replace long source build parameter list

The parameter list to `SourceConfig::build` is growing quite large, and will likely soon have an additional parameter to support the end-to-end acknowledgement framework. To reduce the complexity of this and any further parameters, convert the parameter list into a `struct SourceContext` containing all the parameters.

Ref: https://github.com/timberio/vector/pull/6911#discussion_r607355981