Make it easier to use Snowplow Micro in development

What does this MR do and why?

Related to #331979 (closed).

This simplifies Snowplow Micro setup in development environment using two env variables: SNOWPLOW_MICRO_ENABLE and SNOWPLOW_MICRO_URI, and also sets up groundwork for adding Snowplow Micro to GDK (gitlab-development-kit#1235 (closed)) for an even easier setup.

Setting up Snowplow Micro now doesn't require manual code changes.

This is a variation on !62629 (closed) with options moved to Gitlab::Tracking::Destinations::Snowplow and adding a new destination Gitlab::Tracking::Destinations::SnowplowMicro inheriting from the former, and used when SNOWPLOW_MICRO_ENABLE env is set to 1.

How to set up and validate locally

Follow the updated Snowplow Micro docs for details on how to use it.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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