Allow configuration option to specify default filter labels

[lafriks]

Clear and concise description of the problem

Currently there is no way to limit where to limit agents where pipelines without any filter labels specified to run on.

Suggested solution

Add configuration option for woodpecker server to set default label filters that would be set for pipelines that has no filter set.

This could also be used as alternative to fix #2634

Alternative

As workaround this possible be also implemented in external configuration service but that seems not so straightforward and could be harder to implement

Additional context

Gitlab CI implements similar approach to allow for agents to set option that they are allowed to run "untagged pipelines"

Validations

• Checked that the feature isn't part of the next version already [https://woodpecker-ci.org/faq#which-version-of-woodpecker-should-i-use]
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.