feat(themes): Add email configuration support for themes

[kanikabansal-juspay]

Type of Change

• Bugfix
• New feature
• Enhancement
• Refactoring
• Dependency updates
• Documentation
• CI/CD

Description

Added changes for the emails as per the email theme config.
Changes are done for:

• verify.html
• reset.html
• invite.html
• magic_link.html

Additional Changes

• This PR modifies the API contract
• This PR modifies the database schema
• This PR modifies application configuration/environment variables

Motivation and Context

To add theme customisation to email templates.

How did you test it?

Checklist

• I formatted the code cargo +nightly fmt --all
• I addressed lints thrown by cargo clippy
• I reviewed the submitted code
• I added unit tests for my changes where possible

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
crates/router/src/services/email/assets/reset.html	81% smaller
crates/router/src/services/email/assets/invite.html	80% smaller
crates/router/src/services/email/assets/magic_link.html	80% smaller
crates/router/src/services/email/assets/verify.html	78% smaller
crates/router/src/services/email/types.rs	31% smaller
config/development.toml	Unsupported file format
crates/router/src/consts/user.rs	0% smaller
crates/router/src/core/user.rs	0% smaller

[ThisIsMani]

Pass background color as well. Check other templates as well.