MAINTAINERS.md add etiennehomer to case-server, conversion-server, network-viewer and ws-dependencies

[etiennehomer]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

[etiennehomer]

I'm a gridsuite developer for over 4 years and also contributing to powsybl repositories. Gridsuite is an opensource project and is largely using the powsybl libraries. I'm often involved in the powsybl versions upgrade for gridsuite as a developer or reviewer.

• for case-server and conversion-server, I'm developer in the team of gridsuite in charge of case storage, case import/export and case conversion.
  I participated in :
  powsybl/powsybl-case-server@ed4c357.

• for network-viewer : i was involved in the creation of the repo and the npm package when the network-area-diagram was implemented.
  I participated in :
  powsybl/powsybl-network-viewer@7769c88
  powsybl/powsybl-network-viewer@7298706
  gridsuite/gridstudy-app@0a71666

• for ws-dependencies : this repo is upgraded and released at every powsybl version upgrage. We need some members of gridsuite to have rights on it.
  I released it twice :
  powsybl/powsybl-ws-dependencies@f186cfa
  powsybl/powsybl-ws-dependencies@ea4e3c7