[scraperhelper] Allow scraper controller to be configured to run scrapers in parallel

[dehaansa]

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Description

When investigating an issue in contrib, it was discovered that the scraperhelper did not have a way to run scrapers in parallel and take advantage of the connection pool in the sqlqueryreceiver. This PR adds the ability to configure the scrapercontroller to run either unbounded goroutines to scrape in parallel or a configurable size worker pool for running scrapers in parallel.

Link to tracking issue

Fixes https://github.com/open-telemetry/opentelemetry-collector/issues/13113

Testing

Unit tests updated, but otherwise untested, posting here for initial review while I evaluate in context in contrib.

Documentation

No docs updated, as there's no existing docs for the package.