This PR adds a new config argument called `participants_matrix`, which
allows the user to easily spin up a matrix of EL/CL combos. The `el` and
`cl` keys within this argument can be fully fledged normal
`participants`, so they support the exact same arguments.

For example:

# Example 1
```
participants_matrix:
  el:
  - el_type: geth
  - el_type: besu
  - el_type: reth
  cl:
  - cl_type: nimbus
 ```
 
 
 This config would create the following participants:
 - `nimbus-geth`
 - `nimbus-besu`
 - `nimbus-reth`

# Example 2
```
participants_matrix:
  el:
  - el_type: geth
  - el_type: besu
  - el_type: reth
  cl:
  - cl_type: nimbus
participants:
 - el_type: nethermind
   cl_type: lighthouse
 ```
 This config would create the following participants:
 - `nimbus-geth`
 - `nimbus-besu`
 - `nimbus-reth`
 - `lighthouse-nethermind`
 
 
# Example 3
```
participants_matrix:
  el:
  - el_type: geth
  - el_type: besu
  - el_type: reth
  cl:
  - cl_type: nimbus
     count: 5
 ```
 This config would create the following participants:
 - `5x nimbus-geth`
 - `5x nimbus-besu`
 - `5x nimbus-reth`

---------
Co-authored-by: Barnabas Busa <busa.barnabas@gmail.com>