[Feature] Allow to install Fleet Integration versions other than latest

[breml]

For our corporate setup, we run 3 Elasticsearch Cluster in Elastic Cloud:

• DEV
• QUAL
• PROD

According to the internal guidelines, the Elasticsearch cluster for DEV is always on the most recent version, QUAL is at least one minor version behind and PROD again is at least one minor version behind QUAL.
When we test a setup, we would like to test the exact combination together with the respective version of the Fleet Integrations, that are installed. This means, that we want to test a certain combination of Elasticsearch + Versions of Fleet Intgrations first on DEV, then on QUAL and finally roll out the tested combination to PROD.
Currently, this is not possible with the Elastic Package Registry and with Terraform, since the official Terraform provider refuses to install an older version of a Terraform integration, if a more recent version is available. This is very unfortunate, since it basically forces us to update the Fleet Integrations whenever we update Elasticsearch and we end up in production with a combination of version, that we have never tested before.

Additional context

I first raised this issue with the Elastic support and they asked me to also raise the issue here (and on the Terraform provider: elastic/terraform-provider-elasticstack#858).

Related issue: elastic/terraform-provider-elasticstack#669