feat: support remote_signer in participants_matrix

[barnabasbusa]

Summary

Fixes #1169.

participants_matrix could not configure a remote signer. A remote_signer dimension was already declared in sanity_check.star's PARTICIPANT_MATRIX_PARAMS, but input_parser.star only iterated el/cl/vc and never merged it into participants — so it was dead config. Putting remote-signer fields under cl also failed sanity validation (use_remote_signer is not an attribute).

Changes

• input_parser.star: the matrix builder now also reads the remote_signer dimension and merges each item's keys into the participant (same way vc is merged), wrapped in a for ... else [{}] so it's optional. Defining a non-empty remote_signer entry implies use_remote_signer = true — no separate flag to set.
• README.md: documented the remote_signer matrix dimension with an example, noting that defining an entry enables it automatically and that it requires use_separate_vc: true on the matching cl item.
• .github/tests/remote-signer-matrix.yaml: example/config exercising the new dimension.

Usage

participants_matrix:
  el:
    - el_type: geth
  cl:
    - cl_type: prysm
      use_separate_vc: true
  vc:
    - vc_type: prysm
  remote_signer:
    - remote_signer_type: web3signer
      remote_signer_image: consensys/web3signer:develop

Just defining the remote_signer entry enables it. A remote signer requires use_separate_vc: true on the matching cl item (existing validation in input_parser.star).

kurtosis lint passes.