Run manual pipeline: allow executing a single/specific pipeline if more than one pipeline definitions exist

Clear and concise description of the problem

The "Run pipeline" button in the UI, which is triggered for "event: manual", currently executes all existing pipeline definitions under .woodpecker/.

Suggested solution

The button could allow invoking specific pipelines only instead of starting all existing ones.

Alternative

No response

Additional context

No response

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 Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.