feat: Custom expressions for billing metrics

[stephenlago99]

Context

This pull request contains the implementations for:

• A new drawer for editing Custom expressions for Billable metrics.
• A new section inside billable metrics that allows you to switch between Unique field / Custom expression

The changes were done based on the frontend dive-in: [FE] [Dive-in] Billable metric expressions.

Description

👉 CustomExpressionDrawer:

• Has two editors, enables the user to type & verify & save an expression
• The expression verification uses a new lago-expression package

👉 JsonEditor:

• Refactored to support passing new text modes (this is useful for not validating expressions as JSON). The thought process behind this change can be found in the dive-in.
• Refactored to support some new properties (custom errors, custom validate)

👉 Drawer:

• Refactored to support passing a classname to the StickyBottomBar (in order to be able to pass a z-index, as the editors had a z-index of 4)

👉 lago-expression:

• Currently it is embedded in our application, since the npm release process seemed trickier for a wasm package. When it will be released, we will swap it and remove it from our project.