feat: Allow Environment Variables to override config.json

[dghelm]

Purpose or design rationale of this PR

The goal is to allow the service to be configured using environmental variables (in addition to the config.json file). This facilitates contexts like Kubernetes, where you want Secrets managed independently from the non-sensitive config information (say, loaded from AWS Secrets).

Here, variables are prefixed with SCROLL_CHAIN_MONITOR_.

For more info and context, see scroll-tech/scroll#1489

Sensitive values include:

• SCROLL_CHAIN_MONITOR_DB_CONFIG_DSN

PR title

Your PR title must follow conventional commits (as we are doing squash merge for each PR), so it must start with one of the following types:

• build: Changes that affect the build system or external dependencies (example scopes: yarn, eslint, typescript)
• ci: Changes to our CI configuration files and scripts (example scopes: vercel, github, cypress)
• docs: Documentation-only changes
• feat: A new feature
• fix: A bug fix
• perf: A code change that improves performance
• refactor: A code change that doesn't fix a bug, or add a feature, or improves performance
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc)
• test: Adding missing tests or correcting existing tests

Deployment tag versioning

Has tag in common/version.go been updated or have you added bump-version label to this PR?

• No, this PR doesn't involve a new deployment, git tag, docker image tag
• Yes

Breaking change label

Does this PR have the breaking-change label?

• No, this PR is not a breaking change
• Yes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 40.82%. Comparing base (5accd3b) to head (972fbd1).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #125   +/-   ##
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  Coverage   40.82%   40.82%           
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  Files           5        5           
  Lines         605      605           
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  Hits          247      247           
  Misses        349      349           
  Partials        9        9           

Flag	Coverage Δ
chain-monitor	40.82% <ø> (ø)

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