Remove the need to define host in hints autodiscovery by using proper default values

[ChrsMark]

Describe the enhancement:
At the moment users need to specify the host to collect metrics from a Pod in hints based autodiscovery.
We can make the host setting optional and use as the default the most common value which is the Pod's IP along with the proper container's port. At the moment the user needs to provide sth like ${kubernetes.pod.ip}:3601. With this enhancement, the user would just need to define the package in hints and the defaults should be able to cover in the general case. This enhancement was identified after working on elastic/observability-docs#2198.

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1. [Hints Support] Add default host #3575
   Team:Cloudnative-Monitoring Team:Elastic-Agent backport-skip enhancement +4
   
2. [Hints Support] Update documentation for hints/host ingest-docs#577
   backport-skip +1
   

[mlunadia]

@ChrsMark let's give this a more specific title. Improve
https://github.com/elastic/observability-dev/blob/461a217dc91cb6e099fb7f64b040e8f25bb1ec1c/docs/how-we-work/writing-content-cheatsheet.md#titles-checklist