Add special support for latest version to remote attacher

Our AgentDownloader currently constructs maven URL in the form: https://repo1.maven.org/maven2/co/elastic/apm/elastic-apm-agent/<VERSION>, where the VERSION comes from the input arguments. We want to add special handling to get the latest version. Requirements are:

1. When the requested version is latest - use the following URL for agent downloading: https://oss.sonatype.org/service/local/artifact/maven/redirect?r=releases&g=co.elastic.apm&a=elastic-apm-agent&v=LATEST
2. In the slim CLI, when the version argument is not provided, default to latest.

Related to our Fleet integration - see https://github.com/elastic/kibana/issues/129866 for more context

[eyalkoren]

Oh, you already did? Nice! Do you have a working branch or draft PR that I can have a look at?

I am just finishing tests, related docs and --help instructions and uploading for review

[felixbarny]

I just had another idea. We can also call https://oss.sonatype.org/service/local/artifact/maven/redirect?r=releases&g=co.elastic.apm&a=elastic-apm-agent&p=jar.asc&v=LATEST and not follow the redirect but instead check the Location header manually.