Allow setting name for matrix pipelines

Clear and concise description of the problem

Since #2695 the build status of matrix pipelines gets correctly reported, what is awesome. However, there is still room for improvement. Currently it gets reported as {name}/{number}. It would be better if a name would be used instead of a number.

Suggested solution

Add a new filed matrixname which allows setting the name for a matrix pipeline. With the example from the documentation it could look like this:

matrix:
  GO_VERSION:
    - 1.4
    - 1.3
  DATABASE:
    - mysql:5.5
    - mysql:6.5
    - mariadb:10.1

steps:
  build:
    image: golang:${GO_VERSION}
    commands:
      - go get
      - go build
      - go test

services:
  database:
    image: ${DATABASE}

matrixname: ${GO_VERSION}-${DATABASE}

This would be reported as e.g. {name}/1.4-mysql:5.5.

Alternative

If only one variable is used, we can also use the variable instead of the number.

Additional context

No response

Validations

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

woodpecker-ci / woodpecker