internal/cli: fix db.engine key naming for config files

[manav2401]

Description

Fixes the key for newly added db.engine flag when read by config files (hcl or toml). Also, comments in config file as default value is anyways used.

Changes

• Bugfix (non-breaking change that solves an issue)
• Hotfix (change that solves an urgent issue, and requires immediate attention)
• New feature (non-breaking change that adds functionality)
• Breaking change (change that is not backwards-compatible and/or changes current functionality)
• Changes only for a subset of nodes

Breaking changes

Please complete this section if any breaking changes have been made, otherwise delete it

Nodes audience

In case this PR includes changes that must be applied only to a subset of nodes, please specify how you handled it (e.g. by adding a flag with a default value...)

Checklist

• I have added at least 2 reviewer or the whole pos-v1 team
• I have added sufficient documentation in code
• I will be resolving comments - if any - by pushing each fix in a separate commit and linking the commit hash in the comment reply
• Created a task in Jira and informed the team for implementation in Erigon client (if applicable)
• Includes RPC methods changes, and the Notion documentation has been updated

Cross repository changes

• This PR requires changes to heimdall
  • In case link the PR here:
• This PR requires changes to matic-cli
  • In case link the PR here:

Testing

• I have added unit tests
• I have added tests to CI
• I have tested this code manually on local environment
• I have tested this code manually on remote devnet using express-cli
• I have tested this code manually on mumbai
• I have created new e2e tests into express-cli

Manual tests

Please complete this section with the steps you performed if you ran manual tests for this functionality, otherwise delete it

Additional comments

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[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.01% ⚠️

Comparison is base (66e8296) 57.05% compared to head (21b2b04) 57.04%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #969      +/-   ##
===========================================
- Coverage    57.05%   57.04%   -0.01%     
===========================================
  Files          633      633              
  Lines        77333    77333              
===========================================
- Hits         44121    44117       -4     
- Misses       29454    29457       +3     
- Partials      3758     3759       +1     

Files Changed	Coverage Δ
internal/cli/server/config.go	69.96% <ø> (ø)

... and 14 files with indirect coverage changes

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