ADR for usage of gRPC for networking between services

[cturnbull-bitwarden]

🎟️ Tracking

Originated out of https://bitwarden.atlassian.net/wiki/spaces/EN/pages/809828356/Proposal+The+Pricing+Plans+Service

📔 Objective

This PR introduces a new ADR titled "Adopting gRPC for Inter-Service Communication". The purpose is to document the decision-making process and outcomes regarding adopting gRPC for inter-service communication within Bitwarden's microservices architecture.

⏰ Reminders before review

• Contributor guidelines followed
• All formatters and local linters executed and passed
• Written new unit and / or integration tests where applicable
• Protected functional changes with optionality (feature flags)
• Used internationalization (i18n) for all UI strings
• CI builds passed
• Communicated to DevOps any deployment requirements
• Updated any necessary documentation (Confluence, contributing docs) or informed the documentation team

🦮 Reviewer guidelines

• 👍 (:+1:) or similar for great changes
• 📝 (:memo:) or ℹ️ (:information_source:) for notes or general info
• ❓ (:question:) for questions
• 🤔 (:thinking:) or 💭 (:thought_balloon:) for more open inquiry that's not quite a confirmed issue and could potentially benefit from discussion
• 🎨 (:art:) for suggestions / improvements
• ❌ (:x:) or ⚠️ (:warning:) for more significant problems or concerns needing attention
• 🌱 (:seedling:) or ♻️ (:recycle:) for future improvements or indications of technical debt
• ⛏ (:pick:) for minor or nitpick changes

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[withinfocus]

@cturnbull-bitwarden for what it's worth, it could still be a good idea to finish this and then merge it as rejected -- I'd rather it not disappear as a closed PR.